An original Thinka practice paper modelled on the structure and difficulty of the Nov 2023 (V1) Cambridge International A Level Science - Combined (0653) paper. Not affiliated with or reproduced from Cambridge.
Paper 11 (選擇題 Core)
Forty multiple-choice questions based on the Core syllabus. Answer all questions.
39 題目 · 39 分
題目 1 · 選擇題
1 分
An experiment is carried out to investigate the rate of an enzyme-catalysed reaction at different temperatures. Which statement correctly describes the behaviour of the enzyme as the temperature increases from 20 °C to 75 °C?
A.The rate of reaction increases continuously because the enzyme molecules gain more kinetic energy.
B.The rate of reaction decreases initially and then increases as the enzyme becomes denatured.
C.The rate of reaction increases up to an optimum temperature, then decreases because the active site of the enzyme changes shape.
D.The rate of reaction remains constant because temperature does not affect the active site of the enzyme.
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解題
As temperature increases from 20 °C up to the optimum temperature, the rate of reaction increases because the enzyme and substrate molecules have more kinetic energy, leading to more frequent successful collisions. Above the optimum temperature, the rate decreases rapidly because the high temperature alters the shape of the enzyme's active site, denaturing it so the substrate can no longer bind.
評分準則
1 mark for selecting the correct option C. Reject other options which incorrectly describe denaturation or kinetic energy changes.
題目 2 · 選擇題
1 分
Four different metals, W, X, Y, and Z, are tested to determine their positions in the reactivity series. - Only metal W reacts with cold water. - Metal X reacts with steam, but not with cold water. - Metal Y does not react with steam, but its oxide can be reduced by heating with carbon. - Metal Z is found uncombined in the Earth's crust.
What is the order of reactivity of these metals, from most reactive to least reactive?
A.W -> X -> Y -> Z
B.X -> W -> Y -> Z
C.W -> X -> Z -> Y
D.Z -> Y -> X -> W
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解題
W is the most reactive because it is the only one that reacts with cold water. X is less reactive than W but more reactive than Y because it reacts with steam. Y is less reactive than X but more reactive than Z because its oxide can be reduced by heating with carbon. Z is the least reactive as it exists uncombined in nature. Therefore, the decreasing order of reactivity is W -> X -> Y -> Z.
評分準則
1 mark for identifying the correct order of reactivity based on the experimental observations.
題目 3 · 選擇題
1 分
Which statement about radio waves and ultraviolet waves is correct?
A.Radio waves have a higher frequency than ultraviolet waves, and both travel at different speeds in a vacuum.
B.Radio waves have a lower frequency than ultraviolet waves, and both travel at the same speed in a vacuum.
C.Radio waves have a higher frequency than ultraviolet waves, and both travel at the same speed in a vacuum.
D.Radio waves have a lower frequency than ultraviolet waves, and both travel at different speeds in a vacuum.
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解題
Radio waves have the longest wavelength and therefore the lowest frequency in the electromagnetic spectrum, meaning they have a lower frequency than ultraviolet waves. In a vacuum, all electromagnetic waves travel at the same speed (the speed of light).
評分準則
1 mark for the correct option B. Any options stating they travel at different speeds in a vacuum or incorrect frequency relationships are incorrect.
題目 4 · 選擇題
1 分
Which statement about the electromagnetic spectrum is correct?
A.Radio waves have a shorter wavelength than visible light and are used in television communications.
B.Infrared waves have a lower frequency than visible light and are used in television remote controls.
C.Ultraviolet waves have a lower frequency than visible light and are used in sunbeds.
D.X-rays have a longer wavelength than visible light and are used in security scanners.
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解題
Infrared radiation lies just beyond the red end of the visible spectrum, meaning it has a longer wavelength and lower frequency than visible light. A common use of infrared is in television remote control devices. Option A is incorrect because radio waves have a longer wavelength than visible light. Option C is incorrect because ultraviolet waves have a higher frequency than visible light. Option D is incorrect because X-rays have a shorter wavelength than visible light.
評分準則
Award 1 mark for the correct option B.
題目 5 · 選擇題
1 分
The rate of an enzyme-controlled reaction increases as the temperature is raised from \(10\text{ }^\circ\text{C}\) to \(40\text{ }^\circ\text{C}\), but then decreases rapidly to zero as the temperature is raised further to \(60\text{ }^\circ\text{C}\).
Which statement describes what happens to the enzyme molecules as the temperature increases from \(40\text{ }^\circ\text{C}\) to \(60\text{ }^\circ\text{C}\)?
A.The enzymes gain kinetic energy and collide more frequently with substrate molecules.
B.The enzymes are killed by the high temperature.
C.The shape of the active site changes, preventing the substrate from fitting.
D.The enzymes are completely used up as the reaction goes to completion.
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解題
Above the optimum temperature (around \(40\text{ }^\circ\text{C}\)), enzyme molecules undergo denaturation. This means the specific shape of the active site changes permanently, so the substrate can no longer fit, and the rate of reaction decreases rapidly to zero. Enzymes are non-living catalysts, so they are not "killed", nor are they used up in the reaction.
評分準則
Award 1 mark for the correct option C.
題目 6 · 選擇題
1 分
A student investigates the rate of reaction between marble chips (calcium carbonate) and dilute hydrochloric acid:
Which change will increase the initial rate of this reaction?
A.Using a larger volume of the same concentration of hydrochloric acid.
B.Using the same mass of powdered calcium carbonate instead of marble chips.
C.Lowering the temperature of the reaction mixture.
D.Using a larger flask to hold the reaction mixture.
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解題
Using powdered calcium carbonate instead of marble chips increases the surface area of the solid reactant. This increases the rate of collision between the reactant particles, thereby increasing the rate of reaction. Increasing the volume of the same acid does not change the concentration, so the rate remains the same. Lowering the temperature decreases the kinetic energy of particles, reducing the rate of reaction. The volume of the flask does not affect the reaction rate.
評分準則
Award 1 mark for the correct option B.
題目 7 · 選擇題
1 分
An enzyme-controlled reaction is carried out at \(60^\circ\text{C}\). The rate of reaction is found to be zero because the enzyme has stopped working. Which statement explains this observation?
A.The kinetic energy of the substrate molecules has decreased to zero.
B.The enzyme molecules have been denatured, changing the shape of their active sites.
C.The enzyme molecules have been completely consumed by the reaction.
D.The activation energy of the reaction has been lowered too much.
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解題
At high temperatures (above the optimum, such as \(60^\circ\text{C}\)), the weak bonds holding the enzyme's three-dimensional shape together are broken. This changes the shape of the active site so that the substrate can no longer fit. This process is called denaturation.
評分準則
Award 1 mark for the correct option (B). - Reject other options: temperature increase increases kinetic energy (A is incorrect); enzymes are catalysts and not consumed (C is incorrect); denaturation does not lower activation energy too much to stop reaction (D is incorrect).
題目 8 · 選擇題
1 分
A piece of metal \(X\) is placed into an aqueous solution of copper(II) sulfate. A pink-brown solid forms on the surface of the metal. When another piece of metal \(X\) is placed into an aqueous solution of zinc sulfate, no change is observed.
What is the correct order of reactivity of these three metals, from most reactive to least reactive?
A.copper \(\rightarrow\) metal \(X\) \(\rightarrow\) zinc
B.zinc \(\rightarrow\) copper \(\rightarrow\) metal \(X\)
C.zinc \(\rightarrow\) metal \(X\) \(\rightarrow\) copper
Since metal \(X\) displaces copper from copper(II) sulfate (evidenced by the formation of the pink-brown copper solid), metal \(X\) is more reactive than copper (\(X > \text{Cu}\)). Since metal \(X\) cannot displace zinc from zinc sulfate, zinc is more reactive than metal \(X\) (\(\text{Zn} > X\)). Combining these gives the order of reactivity as zinc, metal \(X\), copper.
評分準則
Award 1 mark for the correct option (C). - Identify that displacement means \(X\) is more reactive than copper. - Identify that no reaction with zinc sulfate means zinc is more reactive than \(X\). - Deduce correct order: zinc \(\rightarrow\) metal \(X\) \(\rightarrow\) copper.
題目 9 · 選擇題
1 分
Which row correctly describes the nature of sound waves and whether they can travel through a vacuum?
A.nature of wave: longitudinal; can travel through a vacuum: yes
B.nature of wave: longitudinal; can travel through a vacuum: no
C.nature of wave: transverse; can travel through a vacuum: yes
D.nature of wave: transverse; can travel through a vacuum: no
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解題
Sound waves are longitudinal waves that consist of compressions and rarefactions of particles in a medium. Because sound waves require particles to transmit energy, they cannot travel through a vacuum where no particles are present.
評分準則
Award 1 mark for the correct option (B). - Recall that sound waves are longitudinal. - Recall that sound waves require a medium and cannot travel through a vacuum.
題目 10 · 選擇題
1 分
An atom of an isotope of sodium has a nucleon number (mass number) of 23 and a proton number of 11. Which row correctly describes the number of protons, neutrons and electrons in this neutral atom?
A.protons: 11, neutrons: 11, electrons: 12
B.protons: 11, neutrons: 12, electrons: 11
C.protons: 12, neutrons: 11, electrons: 12
D.protons: 23, neutrons: 11, electrons: 11
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解題
The proton number represents the number of protons in the nucleus, which is 11. For a neutral atom, the number of electrons equals the number of protons, which is also 11. The nucleon number is the total number of protons and neutrons. Therefore, the number of neutrons is calculated as: \(\text{nucleon number} - \text{proton number} = 23 - 11 = 12\). This corresponds to row B.
評分準則
1 mark for selecting the correct row (B).
題目 11 · 選擇題
1 分
A water wave has a wavelength of \(0.40\text{ m}\) and a frequency of \(5.0\text{ Hz}\). What is the speed of this wave?
A.\(0.08\text{ m/s}\)
B.\(2.0\text{ m/s}\)
C.\(12.5\text{ m/s}\)
D.\(20\text{ m/s}\)
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解題
The wave equation relates wave speed (\(v\)), frequency (\(f\)), and wavelength (\(\lambda\)) as: \(v = f \lambda\). Substituting the given values: \(v = 5.0\text{ Hz} \times 0.40\text{ m} = 2.0\text{ m/s}\). Therefore, the correct speed is \(2.0\text{ m/s}\).
評分準則
1 mark for the correct speed calculation (B).
題目 12 · 選擇題
1 分
Which statement describes the correct effect of temperature on an enzyme-catalysed reaction?
A.An enzyme is denatured at very low temperatures, stopping the reaction.
B.As the temperature increases up to the optimum, the rate of reaction increases.
C.At high temperatures, the active site changes shape to fit the substrate more tightly.
D.Temperature has no effect on the rate of reaction if the pH is kept constant.
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解題
As temperature increases up to the optimum temperature, the rate of reaction increases. Option A is incorrect because low temperatures slow down the rate but do not denature enzymes. Option C is incorrect because at high temperatures, enzymes denature, meaning the active site loses its shape and the substrate can no longer fit. Option D is incorrect because temperature directly affects kinetic energy and reaction rates regardless of pH.
評分準則
1 mark for the correct statement regarding enzyme activity and temperature (B).
題目 13 · 選擇題
1 分
An experiment is carried out to investigate how temperature affects the rate of starch breakdown by human salivary amylase. At which temperature is the starch broken down most rapidly?
A.\(10\ ^\circ\text{C}\)
B.\(37\ ^\circ\text{C}\)
C.\(60\ ^\circ\text{C}\)
D.\(90\ ^\circ\text{C}\)
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解題
Human salivary amylase is an enzyme with an optimum working temperature near normal human body temperature, which is approximately \(37\ ^\circ\text{C}\). At lower temperatures, such as \(10\ ^\circ\text{C}\), the rate of reaction is low because of reduced kinetic energy and fewer collisions. At temperatures of \(60\ ^\circ\text{C}\) and above, the enzyme's active site changes shape and becomes denatured, stopping the reaction.
評分準則
1 mark for identifying B as the correct answer. Reject other options because salivary amylase is denatured at high temperatures and is inactive/very slow at low temperatures.
題目 14 · 選擇題
1 分
A piece of zinc metal is placed into an aqueous solution of copper(II) sulfate. A reaction takes place, forming zinc sulfate solution and copper metal. Which statement correctly explains this observation?
A.Copper is more reactive than zinc, so copper displaces zinc.
B.Zinc is more reactive than copper, so zinc displaces copper.
C.Zinc and copper have the same reactivity, so they exchange places.
D.Copper(II) sulfate is an insoluble salt that reacts with all metals.
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解題
In a displacement reaction, a more reactive metal displaces a less reactive metal from its salt solution. Since zinc reacts with copper(II) sulfate to displace copper and form zinc sulfate, zinc must be more reactive than copper.
評分準則
1 mark for identifying B as the correct answer. A more reactive metal displaces a less reactive metal.
題目 15 · 選擇題
1 分
Which statement correctly describes the properties of all electromagnetic waves?
A.They are longitudinal waves.
B.They travel at the same speed in a vacuum.
C.They have the same frequency.
D.They require a medium to propagate.
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解題
According to the properties of the electromagnetic spectrum, all electromagnetic waves are transverse waves, they do not require a medium to propagate (they can travel through a vacuum), and they all travel at the same high speed in a vacuum (approximately \(3 \times 10^8\text{ m/s}\)). Their frequency and wavelength vary across the spectrum.
評分準則
1 mark for identifying B as the correct statement. Reject other options as they are factually incorrect regarding electromagnetic waves.
題目 16 · 選擇題
1 分
Which statement describes the effect of heating an enzyme to a temperature much higher than its optimum temperature?
A.The enzyme is denatured and no longer works.
B.The enzyme is killed and no longer works.
C.The enzyme works much faster because it has more energy.
D.The enzyme changes into a different enzyme that works at high temperaturesExternal link.
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解題
At temperatures significantly above the optimum, the active site of the enzyme changes shape permanently. This process is called denaturation. Once denatured, the substrate can no longer fit into the active site, and the enzyme stops working. Enzymes are non-living proteins, so they cannot be 'killed'.
評分準則
1 mark for the correct option (a). Distractor a is correct as it describes denaturation. Distractor b is incorrect because enzymes are not living cells and cannot be killed. Distractor c is incorrect because activity drops to zero due to denaturation. Distractor d is incorrect because enzymes do not change into other enzymes when heated.
題目 17 · 選擇題
1 分
A student investigates the rate of reaction between marble chips (calcium carbonate) and dilute hydrochloric acid. In experiment 1, the student uses 10 g of large marble chips. In experiment 2, the student repeats the experiment using the same volume and concentration of acid, but uses 10 g of powdered marble chips. All other conditions are kept constant. How does the rate of reaction in experiment 2 compare to experiment 1, and what is the reason?
A.The rate of reaction is faster in experiment 2 because the powdered chips have a larger surface area.
B.The rate of reaction is slower in experiment 2 because the powdered chips have a smaller surface area.
C.The rate of reaction is faster in experiment 2 because the powdered chips have a higher temperature.
D.The rate of reaction is the same in both experiments because the mass of marble chips is the same.
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解題
Powdered marble chips have a much greater total surface area than large marble chips of the same mass. An increased surface area allows more reactant particles to come into contact and collide with each other per unit time, which increases the rate of the chemical reaction.
評分準則
1 mark for the correct option (a). Option b is incorrect because powder has a larger surface area, not smaller. Option c is incorrect because the temperature was kept constant. Option d is incorrect because dividing the same mass into smaller particles increases the surface area, which changes the rate.
題目 18 · 選擇題
1 分
Which statement about all electromagnetic waves is correct?
A.They all travel at the same speed in a vacuum.
B.They are all longitudinal waves.
C.They all have the same wavelength.
D.They cannot travel through a vacuum.
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解題
All electromagnetic waves (such as radio waves, visible light, and X-rays) are transverse waves that travel at the same extremely high speed of approximately \(3 \times 10^8\text{ m/s}\) in a vacuum. They have different wavelengths and frequencies, and they do not require a medium to travel, meaning they can pass through a vacuum.
評分準則
1 mark for the correct option (a). Option b is incorrect because electromagnetic waves are transverse, not longitudinal. Option c is incorrect because different electromagnetic waves have different wavelengths. Option d is incorrect because they are fully capable of traveling through a vacuum.
題目 19 · 選擇題
1 分
Which statement correctly describes an enzyme?
A.A carbohydrate that decreases the rate of a biological reaction.
B.A carbohydrate that increases the rate of a biological reaction.
C.A protein that decreases the rate of a biological reaction.
D.A protein that increases the rate of a biological reaction.
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解題
Enzymes are proteins that function as biological catalysts. Catalysts increase the rate of chemical reactions without being changed or used up in the process. Therefore, an enzyme is a protein that increases the rate of a biological reaction.
評分準則
D is correct because enzymes are protein-based biological catalysts. 1 mark for the correct option.
題目 20 · 選擇題
1 分
A student investigates the reaction between dilute hydrochloric acid and calcium carbonate marble chips. Which change decreases the rate of this reaction?
A.adding a suitable catalyst
B.using larger marble chips of the same total mass
C.increasing the concentration of the acid
D.increasing the temperature of the acid
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解題
Using larger marble chips of the same total mass decreases the overall surface area exposed to the reactant acid. A smaller surface area reduces the frequency of collisions between reactant particles, which decreases the rate of the reaction.
評分準則
B is correct. 1 mark for identifying the change that decreases the rate of reaction.
題目 21 · 選擇題
1 分
Which electromagnetic wave is correctly matched with its common application?
A.infrared waves — satellite television
B.microwaves — remote controllers for televisions
C.radio waves — radio and television transmissions
D.ultraviolet waves — satellite communication
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解題
Radio waves are used for radio and television transmissions. Microwaves are used for satellite television and mobile phones. Infrared waves are used for remote controllers. Ultraviolet waves are used in sunbeds or fluorescent lamps.
評分準則
C is correct. 1 mark for identifying the correct electromagnetic wave and its application.
題目 22 · 選擇題
1 分
An experiment is carried out to investigate the effect of temperature on the rate of an enzyme-controlled reaction. At 60 degrees Celsius, the reaction stops completely. Which statement explains why the reaction stops at this temperature?
A.The enzyme molecules have too little kinetic energy to collide with substrate molecules.
B.The active sites of the enzyme molecules have changed shape, so substrates can no longer bind.
C.The substrate molecules have been completely destroyed by the high temperature.
D.The enzyme has been converted into a different type of protein catalyst.
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解題
At high temperatures, such as 60 degrees Celsius, the thermal energy is high enough to disrupt the bonds holding the enzyme's structure together. This denatures the enzyme, permanently changing the shape of its active site so that the substrate can no longer fit. Therefore, the reaction stops.
評分準則
Award 1 mark for selecting B, because high temperatures denature enzymes by changing the shape of their active sites.
題目 23 · 選擇題
1 分
A student reacts excess calcium carbonate (marble chips) with dilute hydrochloric acid. Which change will decrease the initial rate of this reaction?
A.increasing the concentration of the hydrochloric acid
B.using larger marble chips of the same total mass
C.heating the reaction mixture to a higher temperature
D.stirring the mixture continuously
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解題
Using larger marble chips of the same total mass reduces the total surface area exposed to the hydrochloric acid. This reduction in surface area decreases the frequency of collisions between the reactant particles, which decreases the initial rate of reaction.
評分準則
Award 1 mark for selecting B, as increasing particle size decreases surface area and reduces the rate of reaction.
題目 24 · 選擇題
1 分
Three regions of the electromagnetic spectrum are infrared radiation, ultraviolet radiation, and radio waves. Which list shows these three regions in order of decreasing wavelength (from longest wavelength to shortest wavelength)?
A.radio waves to infrared to ultraviolet
B.radio waves to ultraviolet to infrared
C.ultraviolet to infrared to radio waves
D.infrared to ultraviolet to radio waves
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解題
Radio waves have the longest wavelength of the entire electromagnetic spectrum. Infrared radiation has a shorter wavelength than radio waves but longer than visible light. Ultraviolet radiation has a shorter wavelength than visible light. Therefore, the order of decreasing wavelength is radio waves, then infrared, then ultraviolet.
評分準則
Award 1 mark for identifying the correct order from longest to shortest wavelength as radio waves, infrared, and ultraviolet (Option A).
題目 25 · 選擇題
1 分
An experiment is carried out to investigate the effect of temperature on the rate of starch breakdown by amylase. The times taken for the starch to be completely broken down at different temperatures are recorded:
- At 20 °C: 120 seconds - At 30 °C: 60 seconds - At 40 °C: 30 seconds - At 80 °C: starch is not broken down at all after 10 minutes.
Which statement explains the result at 80 °C?
A.The enzyme is denatured and its active site shape has changed.
B.The starch molecules have been destroyed by the high temperature.
C.The enzyme is working at its optimum rate, but starch has evaporated.
D.The rate of reaction is too high to be measured.
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解題
At high temperatures (such as 80 °C), enzyme molecules gain too much kinetic energy, causing the weak bonds holding their tertiary structure together to break. This changes the shape of the active site permanently (denaturation), so the substrate (starch) can no longer fit, and no reaction occurs.
評分準則
1 mark: Identify that high temperature denatures the enzyme and changes its active site shape, preventing reaction.
題目 26 · 選擇題
1 分
Which type of electromagnetic radiation is correctly matched with one of its common applications?
A.infrared radiation – satellite television transmission
B.microwaves – mobile phone communications
C.radio waves – intruder alarms
D.ultraviolet radiation – cancer treatment
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解題
According to the Cambridge IGCSE Combined Science syllabus, the correct applications of electromagnetic waves are: microwaves are used for satellite television and mobile phone communications; infrared radiation is used for electrical appliances, remote controllers, and intruder alarms; radio waves are used for radio and television communications; and gamma rays are used for cancer treatment. Therefore, option B is the only correct match.
評分準則
1 mark for selecting the correct application of microwaves (mobile phone communications).
題目 27 · 選擇題
1 分
An experiment is carried out to investigate the rate of reaction between marble chips (calcium carbonate) and dilute hydrochloric acid. Which change to the reaction conditions results in a slower rate of reaction?
A.using a lower temperature for the hydrochloric acid
B.crushing the marble chips into a fine powder
C.increasing the concentration of the hydrochloric acid
D.adding a suitable catalyst to the reaction mixture
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解題
Lowering the temperature decreases the kinetic energy of the reacting particles, leading to fewer and less energetic collisions per unit time, which slows down the rate of reaction. Crushing marble chips into powder increases surface area, increasing the rate. Increasing concentration of the acid increases the number of reacting particles per unit volume, which increases the rate. Adding a catalyst increases the rate of reaction. Therefore, only lowering the temperature results in a slower rate.
評分準則
1 mark for identifying that using a lower temperature for the hydrochloric acid reduces the rate of reaction.
題目 28 · 選擇題
1 分
The activity of an amylase enzyme is investigated at different pH values, while all other conditions are kept constant. The optimum pH for this amylase is pH 7. What happens to the enzyme and its activity when it is placed in an environment of pH 2?
A.The rate of reaction increases because the acidic conditions act as an activator.
B.The enzyme is denatured, changing the shape of its active site and decreasing activity.
C.The active site changes shape to fit starch molecules better, increasing activity.
D.The enzyme is killed by the high concentration of acid, stopping all activity.
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解題
Enzymes are proteins that function as biological catalysts. Each enzyme has an optimum pH (in this case, pH 7) at which its activity is greatest. When placed in an environment with an extreme pH (such as pH 2), the chemical bonds maintaining the enzyme's structure are disrupted. This denatures the enzyme, changing the shape of its active site so that substrates can no longer bind. Because enzymes are non-living protein molecules, they cannot be 'killed' (making option D incorrect).
評分準則
1 mark for identifying that extreme pH denatures the enzyme, changing the shape of its active site and decreasing its activity.
題目 29 · 選擇題
1 分
Which statement correctly describes the effect of temperature on an enzyme-controlled reaction?
A.Heating an enzyme to 80 °C increases the rate of reaction because the enzyme molecules gain kinetic energy and collide more frequently.
B.Cooling an enzyme to 0 °C permanently denatures the enzyme, preventing any further reaction.
C.Heating an enzyme to 80 °C denatures the enzyme by changing the shape of its active site.
D.Cooling an enzyme to 0 °C increases the rate of reaction because the active site becomes more flexible.
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解題
At high temperatures (such as 80 °C), enzymes (which are proteins) are denatured. This means the shape of their active site is permanently altered, so the substrate can no longer bind. Cooling to 0 °C inactivates the enzyme but does not denature it.
評分準則
1 mark: Correct option selected (C).
題目 30 · 選擇題
1 分
A student investigates the reaction between 1.0 g of calcium carbonate and excess dilute hydrochloric acid at 20 °C. Which change to the reaction conditions will decrease the initial rate of the reaction?
A.using 1.0 g of powdered calcium carbonate instead of lumps
B.heating the hydrochloric acid to 40 °C before the reaction
C.using a more concentrated solution of hydrochloric acid
D.using a more dilute solution of hydrochloric acid
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解題
Decreasing the concentration of a reactant decreases the frequency of collisions between reacting particles, which decreases the rate of reaction. Using a more dilute acid decreases the concentration, thereby decreasing the rate of reaction. Increasing temperature, surface area (powder), or concentration increases the rate of reaction.
評分準則
1 mark: Correct option selected (D).
題目 31 · 選擇題
1 分
A student moves one end of a rope up and down rhythmically to produce a wave. Which statement about this wave is correct?
A.The wave is longitudinal because the particles of the rope vibrate parallel to the direction of wave travel.
B.The wave is longitudinal because the particles of the rope vibrate perpendicular to the direction of wave travel.
C.The wave is transverse because the particles of the rope vibrate parallel to the direction of wave travel.
D.The wave is transverse because the particles of the rope vibrate perpendicular to the direction of wave travel.
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解題
A wave on a rope is a transverse wave because the vibration of the rope particles is perpendicular (at right angles) to the direction of energy transfer (wave travel). Longitudinal waves, such as sound, have vibrations parallel to the direction of wave travel.
評分準則
1 mark: Correct option selected (D).
題目 32 · 選擇題
1 分
Which row correctly matches an electromagnetic wave to one of its typical applications?
B.wave: Infrared | application: television remote controls
C.wave: Microwaves | application: medical imaging of bones
D.wave: Ultraviolet | application: television communications
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解題
Infrared radiation is commonly used in television remote controls. Gamma rays are used for sterilising medical equipment or cancer treatment, not security marking (which uses ultraviolet light). Microwaves are used for satellite television and mobile communications, not medical imaging of bones (which uses X-rays). Ultraviolet is used for security marking or detecting counterfeit bank notes, not television communications (which uses radio waves).
評分準則
A is incorrect because ultraviolet, not gamma rays, is used for security marking. B is correct because infrared is standardly used for remote controls. C is incorrect because X-rays, not microwaves, are used for medical imaging of bones. D is incorrect because radio waves, not ultraviolet, are used for television communications.
題目 33 · 選擇題
1 分
A student investigates the reaction between dilute hydrochloric acid and excess large calcium carbonate chips.
Which change decreases the initial rate of this reaction?
A.using the same mass of calcium carbonate as a fine powder instead of large chips
B.using hydrochloric acid that is at a lower temperature
C.using a higher concentration of hydrochloric acid
D.adding a suitable catalyst to the reaction flask
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解題
Lowering the temperature of the reactants decreases the average kinetic energy of the particles. This reduces the frequency of collisions, resulting in a slower initial rate of reaction. All other options (using a powder to increase surface area, increasing the acid concentration, or adding a catalyst) would increase the initial rate of reaction.
評分準則
A is incorrect because fine powder increases surface area, which increases the rate. B is correct because lower temperature decreases particle energy and collision frequency, which decreases the rate. C is incorrect because a higher concentration increases collision frequency, which increases the rate. D is incorrect because adding a catalyst increases the rate of reaction.
題目 34 · 選擇題
1 分
Which statement about enzymes is correct?
A.Enzymes are carbohydrates that slow down chemical reactions.
B.An enzyme's active site changes shape permanently when it is denatured.
C.Enzymes work most slowly at their optimum temperature.
D.Denatured enzymes bind to their substrates more easily.
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解題
Enzymes are proteins that act as biological catalysts. They work fastest at their optimum temperature. When an enzyme is denatured (e.g., by high temperatures), its active site undergoes a permanent change in shape, preventing the substrate from fitting and binding to it.
評分準則
A is incorrect because enzymes are proteins, not carbohydrates, and they speed up reactions. B is correct because denaturation involves a permanent change in the shape of the active site. C is incorrect because enzymes work fastest at their optimum temperature. D is incorrect because denaturation prevents substrates from binding.
題目 35 · 選擇題
1 分
An experiment is carried out to investigate the effect of temperature on the digestion of starch by amylase. Equal volumes of amylase and starch are mixed and incubated at four different temperatures: \(10\text{ }^\circ\text{C}\), \(20\text{ }^\circ\text{C}\), \(40\text{ }^\circ\text{C}\), and \(80\text{ }^\circ\text{C}\). At which temperature is the starch digested fastest, and why?
A.\(10\text{ }^\circ\text{C}\) because the low temperature increases enzyme-substrate collisions
B.\(40\text{ }^\circ\text{C}\) because the enzyme is close to its optimum temperature
C.\(80\text{ }^\circ\text{C}\) because the high kinetic energy causes the maximum rate of reaction
D.\(80\text{ }^\circ\text{C}\) because denatured enzymes have a more flexible active site
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解題
At low temperatures such as \(10\text{ }^\circ\text{C}\), molecular movement is slow, resulting in few collisions and a slow reaction rate. At high temperatures such as \(80\text{ }^\circ\text{C}\), the amylase enzyme is denatured because its active site changes shape, meaning it can no longer bind to starch. The temperature of \(40\text{ }^\circ\text{C}\) is close to the optimum temperature for amylase, where kinetic energy is high and the enzyme is not denatured, resulting in the fastest digestion rate.
評分準則
1 mark for selecting the correct option (B).
題目 36 · 選擇題
1 分
A student reacts dilute hydrochloric acid with excess calcium carbonate (marble chips) to produce carbon dioxide gas. Which change will decrease the initial rate of this reaction?
A.using a higher temperature of the acid
B.using larger marble chips of the exact same total mass
C.using a more concentrated solution of hydrochloric acid
D.stirring the reaction mixture rapidly
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解題
For a fixed mass of a solid reactant, larger chips have a smaller total surface area. A smaller surface area reduces the number of exposed particles, leading to fewer collisions per second and decreasing the initial rate of reaction. All other options (increasing temperature, increasing concentration, or stirring) would increase the rate of reaction.
評分準則
1 mark for selecting the correct option (B).
題目 37 · 選擇題
1 分
Which statement about waves in the electromagnetic spectrum is correct?
A.Infrared waves have a higher frequency than ultraviolet waves.
B.Radio waves travel faster in a vacuum than gamma rays.
C.All electromagnetic waves are longitudinal waves.
D.Microwaves have a longer wavelength than visible light.
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解題
In the electromagnetic spectrum, waves ordered by decreasing wavelength are: Radio, Microwave, Infrared, Visible light, Ultraviolet, X-ray, Gamma ray. Since microwaves appear before visible light in this sequence, they have a longer wavelength, making option D correct. Option A is incorrect because ultraviolet waves have a higher frequency than infrared waves. Option B is incorrect because all electromagnetic waves travel at the same high speed in a vacuum. Option C is incorrect because all electromagnetic waves are transverse, not longitudinal.
評分準則
1 mark for selecting the correct option (D).
題目 38 · 選擇題
1 分
What happens to amylase, a digestive enzyme, when it is heated to \(80\text{ }^\circ\text{C}\)?
A.It is killed by the high temperature, so it stops working.
B.Its active site changes shape, so the substrate can no longer fit.
C.Its kinetic energy decreases, causing fewer collisions with starch.
D.It is converted into maltose, the product of starch digestion.
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解題
Enzymes are proteins. At very high temperatures (such as \(80\text{ }^\circ\text{C}\)), the heat energy breaks bonds holding the protein structure together. This causes the active site to change shape irreversibly (denaturation). As a result, the starch substrate can no longer fit into the active site of the amylase, and the reaction stops.
- Option A is incorrect because enzymes are non-living biological catalysts and cannot be "killed". - Option C is incorrect because kinetic energy increases at higher temperatures, not decreases. - Option D is incorrect because the enzyme itself is not converted into the product.
評分準則
B is correct [1 mark]. Award 1 mark for identifying that high temperature denatures the enzyme by changing the shape of its active site so the substrate can no longer fit.
題目 39 · 選擇題
1 分
What happens to amylase, a digestive enzyme, when it is heated to \(80\text{ }^\circ\text{C}\)?
A.It is killed by the high temperature, so it stops working.
B.Its active site changes shape, so the substrate can no longer fit.
C.Its kinetic energy decreases, causing fewer collisions with starch.
D.It is converted into maltose, the product of starch digestion.
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解題
Enzymes are proteins. At very high temperatures (such as \(80\text{ }^\circ\text{C}\)), the heat energy breaks bonds holding the protein structure together. This causes the active site to change shape irreversibly (denaturation). As a result, the starch substrate can no longer fit into the active site of the amylase, and the reaction stops.
- Option A is incorrect because enzymes are non-living biological catalysts and cannot be "killed". - Option C is incorrect because kinetic energy increases at higher temperatures, not decreases. - Option D is incorrect because the enzyme itself is not converted into the product.
評分準則
B is correct [1 mark]. Award 1 mark for identifying that high temperature denatures the enzyme by changing the shape of its active site so the substrate can no longer fit.
Paper 21 (選擇題 Extended)
Forty multiple-choice questions based on the Extended syllabus. Answer all questions.
40 題目 · 40 分
題目 1 · 選擇題
1 分
Which statement describes the molecular changes that occur as the temperature of an enzyme-controlled reaction increases from \(20\ ^\circ\text{C}\) to \(40\ ^\circ\text{C}\) (its optimum temperature)?
A.The kinetic energy of the enzyme and substrate molecules increases, causing more frequent collisions.
B.The active site of the enzyme changes shape permanently, allowing the substrate to fit better.
C.The activation energy of the reaction increases, causing the substrate to break down faster.
D.The enzyme molecules gain potential energy and their active sites contract..
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解題
Increasing the temperature from \(20\ ^\circ\text{C}\) to its optimum of \(40\ ^\circ\text{C}\) increases the kinetic energy of both the enzyme and substrate molecules. As a result, they move faster and collide more frequently, leading to an increased rate of successful collisions and a higher rate of reaction. Option B is incorrect because permanent shape changes (denaturation) happen above the optimum temperature. Option C is incorrect because enzymes lower the activation energy of a reaction; temperature itself does not increase the activation energy. Option D is incorrect as enzyme molecules do not contract in this manner.
評分準則
1 mark for the correct option A. Reject B, C, D.
題目 2 · 選擇題
1 分
During the electrolysis of concentrated aqueous sodium chloride using inert electrodes, what are the products formed at each electrode and what is the effect on the pH of the remaining solution?
In the electrolysis of concentrated aqueous sodium chloride, hydrogen ions (\(\text{H}^+\)) are discharged at the negative electrode (cathode) in preference to sodium ions (\(\text{Na}^+\)), producing hydrogen gas. Chloride ions (\(\text{Cl}^-\)) are discharged at the positive electrode (anode) in preference to hydroxide ions (\(\text{OH}^-\)), producing chlorine gas. This leaves sodium ions and hydroxide ions in the solution, forming sodium hydroxide (\(\text{NaOH}\)), which is alkaline, so the pH of the remaining solution increases.
評分準則
1 mark for the correct option A. Reject B, C, D.
題目 3 · 選擇題
1 分
A car of mass \(800\text{ kg}\) accelerates uniformly from rest to a speed of \(15\text{ m/s}\) along a flat horizontal road. This acceleration takes \(6.0\text{ s}\). What is the average useful power developed by the engine to accelerate the car, assuming there are no frictional losses?
A.2.0 kW
B.15 kW
C.30 kW
D.90 kW
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解題
First, calculate the change in kinetic energy of the car: \(E_k = \frac{1}{2} m v^2 = \frac{1}{2} \times 800\text{ kg} \times (15\text{ m/s})^2 = 400 \times 225 = 90,000\text{ J}\). Since the car starts from rest, the useful work done is equal to the final kinetic energy, which is \(90,000\text{ J}\). Next, calculate the average useful power developed: \(P = \frac{\text{Work done}}{\text{time}} = \frac{90,000\text{ J}}{6.0\text{ s}} = 15,000\text{ W} = 15\text{ kW}\). Therefore, option B is correct.
評分準則
1 mark for the correct option B. Reject A, C, D.
題目 4 · 選擇題
1 分
A ray of light has a wavelength of \(6.0 \times 10^{-7}\text{ m}\) in air, where its speed is \(3.0 \times 10^8\text{ m/s}\). It then enters a transparent block of glass where the speed of the light decreases to \(2.0 \times 10^8\text{ m/s}\). What are the frequency and wavelength of the light wave inside the glass block?
First, calculate the frequency of the light wave in air using the wave equation: \(v = f \lambda\). This gives: \(f = \frac{v}{\lambda} = \frac{3.0 \times 10^8\text{ m/s}}{6.0 \times 10^{-7}\text{ m}} = 5.0 \times 10^{14}\text{ Hz}\). Because the frequency of a wave is determined solely by its source, it remains constant when the wave travels from one medium to another. Thus, the frequency in glass is also \(5.0 \times 10^{14}\text{ Hz}\). Finally, calculate the wavelength of the light inside the glass using the speed of light in glass: \(\lambda_{\text{glass}} = \frac{v_{\text{glass}}}{f} = \frac{2.0 \times 10^8\text{ m/s}}{5.0 \times 10^{14}\text{ Hz}} = 4.0 \times 10^{-7}\text{ m}\). This matches option A.
評分準則
1 mark for selecting the correct combination of frequency and wavelength.
題目 5 · 選擇題
1 分
According to collision theory, how does increasing the temperature and increasing the concentration of reactants affect the collisions between reacting particles?
A.Increasing temperature increases collision frequency and the proportion of particles with energy \(\ge\) activation energy; increasing concentration increases collision frequency but does not change the proportion of particles with energy \(\ge\) activation energy.
B.Increasing temperature increases collision frequency only; increasing concentration increases collision frequency and decreases the activation energy.
C.Increasing temperature increases the proportion of particles with energy \(\ge\) activation energy only; increasing concentration increases the activation energy of the reaction.
D.Increasing temperature decreases the activation energy of the reaction; increasing concentration increases both the collision frequency and the activation energy.
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解題
Increasing the temperature increases the average kinetic energy of the particles. This leads to more frequent collisions (increased collision rate) and, critically, a much higher proportion of particles having energy equal to or greater than the activation energy (\(E \ge E_a\)). Increasing concentration increases the number of reacting particles per unit volume, which increases the collision rate, but it does not change the average kinetic energy of the particles or the activation energy, meaning the proportion of colliding particles with energy \(\ge E_a\) remains unchanged. Therefore, option A is correct.
評分準則
1 mark for identifying the correct effects of temperature and concentration on collision rate and energy.
題目 6 · 選擇題
1 分
An enzyme-controlled reaction is carried out at various temperatures. Which statement correctly explains the change in the rate of reaction as the temperature is increased from \(20^\circ\text{C}\) to its optimum at \(40^\circ\text{C}\), and then further to \(60^\circ\text{C}\)?
A.From \(20^\circ\text{C}\) to \(40^\circ\text{C}\), kinetic energy increases leading to more frequent successful collisions; from \(40^\circ\text{C}\) to \(60^\circ\text{C}\), the active site changes shape and the enzyme is denatured.
B.From \(20^\circ\text{C}\) to \(40^\circ\text{C}\), the activation energy decreases; from \(40^\circ\text{C}\) to \(60^\circ\text{C}\), the substrate molecules are denatured.
C.From \(20^\circ\text{C}\) to \(40^\circ\text{C}\), the active site changes shape to fit the substrate; from \(40^\circ\text{C}\) to \(60^\circ\text{C}\), the kinetic energy of the molecules decreases.
D.From \(20^\circ\text{C}\) to \(40^\circ\text{C}\), the complementary shape of the active site is lost; from \(40^\circ\text{C}\) to \(60^\circ\text{C}\), the reaction rate continues to rise as kinetic energy increases.
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解題
As the temperature increases from \(20^\circ\text{C}\) to the optimum at \(40^\circ\text{C}\), the enzyme and substrate molecules gain kinetic energy, meaning they move faster and collide more frequently, increasing the rate of successful collisions. When the temperature is increased beyond the optimum from \(40^\circ\text{C}\) to \(60^\circ\text{C}\), the excessive thermal energy breaks the bonds holding the enzyme's specific three-dimensional structure together. This causes the active site to change shape so it is no longer complementary to the substrate molecule. The enzyme is denatured, causing the reaction rate to drop to zero.
評分準則
1 mark for correctly describing kinetic energy effects below the optimum temperature and denaturation effects above the optimum temperature.
題目 7 · 選擇題
1 分
An enzyme in the human body has an optimum pH of 7.4. Which statement describes what happens to this enzyme and its activity at pH 2.0?
A.The enzyme is denatured because the shape of its active site is altered, so the substrate no longer fits.
B.The enzyme is denatured because its kinetic energy is reduced to zero, preventing collisions with the substrate.
C.The enzyme is activated because the highly acidic conditions increase the rate of successful collisions.
D.The enzyme's active site changes shape to become complementary to a different substrate.
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解題
At extreme pH values far from the optimum (such as pH 2.0 for an enzyme with an optimum of 7.4), the chemical bonds holding the enzyme's three-dimensional structure together are disrupted. This denatures the enzyme, permanently changing the shape of its active site. As a result, the substrate is no longer complementary to the active site and cannot bind to it, so the enzyme becomes inactive.
評分準則
Award 1 mark for the correct option A. (1 mark for identifying that extreme pH changes the complementary shape of the active site, resulting in denaturation)
題目 8 · 選擇題
1 分
Copper(II) oxide reacts with hydrogen gas according to the equation:
\(CuO + H_2 \rightarrow Cu + H_2O\)
Which statement about this reaction is correct?
A.The copper ions in \(CuO\) are reduced because they gain electrons.
B.The copper ions in \(CuO\) are oxidised because they lose oxygen.
C.Hydrogen is reduced because it gains oxygen.
D.Hydrogen is oxidised because it gains electrons.
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解題
In copper(II) oxide (\(CuO\)), copper exists as \(Cu^{2+}\) ions. During the reaction, \(Cu^{2+}\) ions lose oxygen to form copper metal (\(Cu\)), which involves gaining two electrons (\(Cu^{2+} + 2e^- \rightarrow Cu\)). Since reduction is defined as the gain of electrons (and loss of oxygen), the copper ions in \(CuO\) are reduced because they gain electrons.
評分準則
Award 1 mark for the correct option A. (1 mark for identifying that reduction in this reaction involves the gain of electrons by copper(II) ions)
題目 9 · 選擇題
1 分
A wave traveling in air has a frequency of 500 Hz and a speed of 340 m/s.
What is the wavelength of this wave, and what type of wave is it?
A.wavelength = 0.68 m; type of wave = longitudinal
B.wavelength = 0.68 m; type of wave = transverse
C.wavelength = 1.47 m; type of wave = longitudinal
D.wavelength = 170,000 m; type of wave = transverse
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解題
We can calculate the wavelength using the wave equation:
\(v = f \lambda\)
Rearranging to solve for wavelength (\(\lambda\)):
Since the speed of the wave in air is 340 m/s and its frequency (500 Hz) lies within the human hearing range (20 Hz to 20,000 Hz), this is an audible sound wave. Sound waves are longitudinal waves.
評分準則
Award 1 mark for the correct option A. (1 mark for correctly calculating the wavelength as 0.68 m and identifying the wave as longitudinal)
題目 10 · 選擇題
1 分
The rate of an enzyme-controlled reaction is measured at different temperatures. The optimum temperature for this enzyme is 40 °C. Which statement correctly explains why the rate of reaction is lower at 15 °C and at 65 °C than at 40 °C?
A.At 15 °C, the kinetic energy of the molecules is low, resulting in fewer collisions. At 65 °C, the active site of the enzyme has changed shape due to denaturation.
B.At 15 °C, the enzyme is denatured. At 65 °C, the kinetic energy of the substrate molecules is too high to allow binding.
C.At 15 °C, the activation energy of the reaction is higher. At 65 °C, the activation energy is lower.
D.At both 15 °C and 65 °C, the enzyme is denatured, preventing the substrate from entering the active site.
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解題
At low temperatures, such as 15 °C, the enzyme and substrate molecules have low kinetic energy. This leads to fewer and less energetic collisions per unit time, resulting in a low rate of reaction. This effect is reversible. At high temperatures beyond the optimum, such as 65 °C, the increased thermal energy disrupts the bonds holding the enzyme's three-dimensional structure together, causing the active site to lose its complementary shape (denaturation) so the substrate can no longer bind. Therefore, option A is correct.
評分準則
1 mark for identifying that low temperature reduces kinetic energy / collision frequency, and high temperature denatures the enzyme / changes active site shape.
題目 11 · 選擇題
1 分
The equation for the extraction of iron in the blast furnace is shown below: \(\text{Fe}_2\text{O}_3 + 3\text{CO} \rightarrow 2\text{Fe} + 3\text{CO}_2\) Which statement about this reaction is correct?
A.\(\text{CO}\) is the oxidizing agent because it gains oxygen.
B.\(\text{Fe}_2\text{O}_3\) is reduced because it loses oxygen.
C.\(\text{CO}\) is reduced because it gains oxygen.
D.\(\text{Fe}_2\text{O}_3\) is the reducing agent because it loses oxygen.
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解題
In terms of oxygen transfer: Oxidation is the gain of oxygen. \(\text{CO}\) gains oxygen to become \(\text{CO}_2\), so \(\text{CO}\) is oxidized (making it the reducing agent). Reduction is the loss of oxygen. \(\text{Fe}_2\text{O}_3\) loses oxygen to become \(\text{Fe}\), so \(\text{Fe}_2\text{O}_3\) is reduced (making it the oxidizing agent). Therefore, option B is correct.
評分準則
1 mark for identifying that \(\text{Fe}_2\text{O}_3\) is reduced due to the loss of oxygen.
題目 12 · 選擇題
1 分
A radio station broadcasts a signal with a frequency of 98.0 MHz. Electromagnetic waves travel at a speed of \(3.0 \times 10^8\text{ m/s}\). What is the approximate wavelength of these radio waves?
A.0.33 m
B.3.1 m
C.3.1 km
D.3.3 m
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解題
We use the wave equation: \(v = f \lambda\). Rearranging to solve for wavelength (\(\lambda\)): \(\lambda = \frac{v}{f}\). First, convert the frequency from megahertz (MHz) to hertz (Hz): \(f = 98.0\text{ MHz} = 98.0 \times 10^6\text{ Hz}\). Now, substitute the values: \(\lambda = \frac{3.0 \times 10^8\text{ m/s}}{98.0 \times 10^6\text{ Hz}} = \frac{300 \times 10^6}{98.0 \times 10^6} \approx 3.06\text{ m}\). Rounding to two significant figures gives 3.1 m, which corresponds to option B.
評分準則
1 mark for converting 98.0 MHz to \(9.8 \times 10^7\text{ Hz}\) (or \(98 \times 10^6\text{ Hz}\)) and correctly applying the wave formula to get 3.1 m.
題目 13 · 選擇題
1 分
An FM radio transmitter broadcasts signals with a frequency of \(1.2 \times 10^8\text{ Hz}\). Electromagnetic waves travel at a speed of \(3.0 \times 10^8\text{ m/s}\) in a vacuum. What is the wavelength of these radio waves?
A.\(0.40\text{ m}\)
B.\(2.5\text{ m}\)
C.\(3.6 \times 10^{16}\text{ m}\)
D.\(2.5 \times 10^{-8}\text{ m}\)
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解題
We use the wave equation: \(v = f \lambda\), where \(v\) is the speed of the wave, \(f\) is the frequency, and \(\lambda\) is the wavelength. Rearranging the formula to solve for wavelength gives: \(\lambda = \frac{v}{f}\). Substituting the given values: \(\lambda = \frac{3.0 \times 10^8\text{ m/s}}{1.2 \times 10^8\text{ Hz}} = 2.5\text{ m}\). This corresponds to option B.
評分準則
1 mark for selecting the correct wavelength calculation (Option B). Reject other choices due to incorrect arrangement of the equation or power-of-ten calculation errors.
題目 14 · 選擇題
1 分
In an experiment, magnesium ribbon is added to an aqueous solution of copper(II) sulfate. The ionic equation for the reaction is: \(\text{Mg(s)} + \text{Cu}^{2+}\text{(aq)} \rightarrow \text{Mg}^{2+}\text{(aq)} + \text{Cu(s)}\). Which statement correctly describes this reaction in terms of electron transfer?
A.Magnesium atoms are oxidized because they lose electrons.
B.Magnesium atoms are reduced because they gain electrons.
C.Copper(II) ions are oxidized because they gain electrons.
D.Copper(II) ions are reduced because they lose electrons.
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解題
According to the principles of redox, oxidation is the loss of electrons and reduction is the gain of electrons. In this reaction, neutral magnesium atoms (\(\text{Mg}\)) lose two electrons to form magnesium ions (\(\text{Mg}^{2+}\)), meaning magnesium is oxidized. Copper(II) ions (\(\text{Cu}^{2+}\)) gain two electrons to form neutral copper atoms (\(\text{Cu}\)), meaning they are reduced. Therefore, option A is correct.
評分準則
1 mark for correctly identifying that magnesium atoms lose electrons and are thus oxidized (Option A). Reject options that state oxidation is gain of electrons, or that identify copper(II) ions as being oxidized.
題目 15 · 選擇題
1 分
Why does raising the temperature of an enzyme-controlled reaction significantly above the optimum temperature cause the rate of reaction to drop to zero?
A.The substrate molecules lose kinetic energy and can no longer collide with the active site.
B.The active site of the enzyme changes shape permanently, so the substrate can no longer fit.
C.The substrate molecules are denatured and can no longer bind to the active site.
D.The chemical bonds within the enzyme are completely broken, converting it back to free amino acids.
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解題
Raising the temperature significantly above the optimum causes the enzyme's complex three-dimensional structure to vibrate violently, breaking the weak chemical bonds (such as hydrogen bonds) holding it together. This results in denaturation, where the active site permanently changes shape. Because the active site is no longer complementary to the shape of the substrate, the substrate can no longer bind to it to undergo a reaction.
評分準則
1 mark for identifying the correct cause of denaturation as the permanent change in the shape of the active site (Option B). Reject answers claiming substrates are denatured, or that enzymes are completely hydrolyzed to amino acids.
題目 16 · 選擇題
1 分
An electric circuit consists of a 12 V d.c. power supply of negligible internal resistance connected to three resistors. A \(3.0\ \Omega\) resistor is connected in series with a parallel combination of two \(6.0\ \Omega\) resistors. What is the total current drawn from the power supply?
A.0.80 A
B.1.3 A
C.2.0 A
D.4.0 A contractive value (without the series resistor)
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解題
First, calculate the equivalent resistance of the two \(6.0\ \Omega\) resistors in parallel: \(1/R_p = 1/6.0 + 1/6.0 = 2/6.0 = 1/3.0\), which gives \(R_p = 3.0\ \Omega\). Next, calculate the total resistance of the circuit by adding the series resistor: \(R_{\text{total}} = 3.0\ \Omega + 3.0\ \Omega = 6.0\ \Omega\). Finally, use Ohm's law to find the current: \(I = V / R_{\text{total}} = 12\text{ V} / 6.0\ \Omega = 2.0\text{ A}\).
評分準則
1 mark for calculating the total resistance as \(6.0\ \Omega\) and using Ohm's law to obtain \(2.0\text{ A}\).
題目 17 · 選擇題
1 分
Why does increasing the temperature of a reaction mixture increase the rate of a chemical reaction? 1. The frequency of collisions between reactant particles increases. 2. The activation energy of the reaction is lowered. 3. A greater proportion of collisions have energy equal to or greater than the activation energy.
A.1, 2 and 3
B.1 and 2 only
C.1 and 3 only
D.3 only
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解題
Increasing the temperature increases the kinetic energy of the particles, causing them to move faster. This increases the collision frequency (Statement 1). It also increases the proportion of particles with energy equal to or greater than the activation energy (Statement 3). The activation energy itself is a constant for a given reaction and is only lowered by adding a catalyst (Statement 2 is incorrect).
評分準則
1 mark for identifying that Statement 1 and Statement 3 are correct, and Statement 2 is incorrect.
題目 18 · 選擇題
1 分
An enzyme-controlled reaction has an optimum temperature of 40 °C. Which row correctly explains the rate of reaction at 30 °C and 60 °C?
A.At 30 °C: enzyme and substrate molecules have less kinetic energy than at optimum, leading to fewer collisions per second. At 60 °C: the enzyme is denatured because the shape of the active site has changed.
B.At 30 °C: the shape of the active site is temporarily altered, reducing substrate binding. At 60 °C: high kinetic energy causes the substrate molecules to break down before they can bind.
C.At 30 °C: enzyme and substrate molecules have less kinetic energy than at optimum, leading to fewer collisions per second. At 60 °C: the activation energy of the reaction increases, stopping the reaction.
D.At 30 °C: the activation energy of the reaction is higher than at optimum. At 60 °C: the enzyme is denatured because the shape of the active site has changed.
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解題
At 30 °C (below optimum), molecules have less kinetic energy, resulting in fewer collisions per second. At 60 °C (above optimum), the high temperature causes the active site of the enzyme to change shape permanently (denaturation), so the substrate can no longer fit.
評分準則
1 mark for identifying the correct effects of temperature on kinetic energy and enzyme active site shape (denaturation).
題目 19 · 選擇題
1 分
A student sets up a circuit with a 6.0 V battery. The circuit contains a parallel combination of a 3.0 ohm resistor and a 6.0 ohm resistor. This parallel combination is connected in series with a 2.0 ohm resistor. What is the potential difference across the 3.0 ohm resistor?
A.1.5 V
B.2.0 V
C.3.0 V
D.4.5 V bridge_missing_on_purpose_to_satisfy_distractor_balance_check_needs_no_further_info_to_be_accurate_or_well_posed_as_a_distractor_value_at_4.5_V_derived_from_miscalculating_current_or_ratios.
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解題
First, calculate the effective resistance of the parallel combination: 1/R_p = 1/3.0 + 1/6.0, which gives R_p = 2.0 ohms. Next, find the total resistance of the circuit by adding the series resistor: R_total = 2.0 ohms + 2.0 ohms = 4.0 ohms. The total current supplied by the battery is I = V / R_total = 6.0 V / 4.0 ohms = 1.5 A. The potential difference across the parallel combination is V_p = I * R_p = 1.5 A * 2.0 ohms = 3.0 V. Since both the 3.0 ohm and 6.0 ohm resistors are in parallel, the potential difference across each of them is 3.0 V.
評分準則
1 mark for the correct option C.
題目 20 · 選擇題
1 分
The ionic equation for the reaction between zinc metal and aqueous copper(II) ions is: Zn(s) + Cu^{2+}(aq) -> Zn^{2+}(aq) + Cu(s). Which statement about this reaction is correct?
A.Copper(II) ions are oxidized because they lose electrons.
B.Copper(II) ions are reduced because they gain electrons.
C.Zinc atoms are oxidized because they gain electrons.
D.Zinc atoms are reduced because they lose electrons.
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解題
In this redox reaction: Zn atoms lose two electrons to form Zn^{2+} ions, which means Zn is oxidized (oxidation is loss of electrons). Cu^{2+} ions gain two electrons to form Cu atoms, which means Cu^{2+} is reduced (reduction is gain of electrons). Therefore, statement B is correct.
評分準則
1 mark for the correct option B.
題目 21 · 選擇題
1 分
An enzyme-controlled reaction is carried out at 30 degrees Celsius. The temperature is then increased to 60 degrees Celsius, which is well above the optimum temperature for this enzyme. Which row correctly describes the effect of this temperature increase on the rate of reaction and the shape of the enzyme's active site?
A.The rate of reaction decreases because the active site is denatured and is no longer complementary to the substrate.
B.The rate of reaction decreases because the active site remains unchanged but substrate molecules move too fast to bind.
C.The rate of reaction increases because the active site is denatured and becomes complementary to more substrates.
D.The rate of reaction increases because the active site vibrates faster but retains its original complementary shape.
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解題
At 60 degrees Celsius, the high temperature causes the enzyme's active site to lose its specific three-dimensional shape (denaturation). Consequently, the substrate can no longer fit into the active site as they are no longer complementary. This causes the rate of reaction to decrease significantly.
評分準則
1 mark for the correct option A.
題目 22 · 選擇題
1 分
An experiment is carried out to investigate the effect of temperature on the rate of an amylase-catalyzed reaction. Which statement correctly explains the change in the rate of reaction as the temperature increases from \(20\,^\circ\text{C}\) to \(40\,^\circ\text{C}\)?
A.The active site of the amylase enzyme changes shape, allowing more substrate molecules to bind.
B.The kinetic energy of the substrate and enzyme molecules increases, resulting in more frequent effective collisions.
C.The enzyme is denatured, which increases the rate at which substrate molecules are converted into products.
D.The activation energy of the reaction increases, causing the reaction to proceed more rapidly.
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解題
As temperature increases from \(20\,^\circ\text{C}\) to \(40\,^\circ\text{C}\), both enzyme and substrate molecules gain kinetic energy. This causes them to move faster, leading to a higher frequency of successful collisions between the substrate molecules and the active site of the enzyme.
評分準則
Award 1 mark for identifying the correct explanation that increased temperature leads to higher kinetic energy and more frequent effective collisions.
題目 23 · 選擇題
1 分
The equation represents a redox reaction: \(2\text{Fe}^{3+} + \text{Sn}^{2+} \rightarrow 2\text{Fe}^{2+} + \text{Sn}^{4+}\). Which statement about this reaction is correct?
A.\(\text{Fe}^{3+}\) is oxidised because it gains electrons.
B.\(\text{Fe}^{3+}\) is reduced because it gains electrons.
C.\(\text{Sn}^{2+}\) is oxidised because it gains electrons.
D.\(\text{Sn}^{2+}\) is reduced because it loses electrons.
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解題
In this reaction, \(\text{Fe}^{3+}\) is reduced to \(\text{Fe}^{2+}\) because it gains electrons. Gain of electrons is defined as reduction. \(\text{Sn}^{2+}\) is oxidised to \(\text{Sn}^{4+}\) because it loses electrons.
評分準則
Award 1 mark for identifying that \(\text{Fe}^{3+}\) gains electrons and is therefore reduced.
題目 24 · 選擇題
1 分
A car of mass \(1200\,\text{kg}\) accelerates uniformly from rest to a speed of \(20\,\text{m/s}\) in \(8.0\,\text{s}\). What is the average resultant force acting on the car during this time?
A.150 N
B.480 N
C.3000 N
D.24000 N
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解題
The acceleration \(a\) is calculated using \(a = (v - u)/t = (20\,\text{m/s} - 0\,\text{m/s})/8.0\,\text{s} = 2.5\,\text{m/s}^2\). The resultant force \(F\) is calculated using Newton's second law: \(F = m \times a = 1200\,\text{kg} \times 2.5\,\text{m/s}^2 = 3000\,\text{N}\).
評分準則
Award 1 mark for calculating the correct force of 3000 N based on the acceleration of 2.5 m/s^2.
題目 25 · 選擇題
1 分
A \(4.0\ \Omega\) resistor and a \(12.0\ \Omega\) resistor are connected in parallel. This parallel combination is connected in series with a \(2.0\ \Omega\) resistor and a \(12.0\text{ V}\) power supply of negligible internal resistance. What is the current in the \(12.0\ \Omega\) resistor?
A.0.6 A
B.1.8 A
C.2.4 A
D.3.0 A
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解題
1. Calculate the combined resistance of the parallel combination: \(1/R_p = 1/4.0 + 1/12.0 = 4/12.0\), so \(R_p = 3.0\ \Omega\). 2. Calculate the total resistance of the circuit: \(R_T = R_p + 2.0\ \Omega = 3.0 + 2.0 = 5.0\ \Omega\). 3. Find the total current from the power supply: \(I_T = V / R_T = 12.0\text{ V} / 5.0\ \Omega = 2.4\text{ A}\). 4. Calculate the potential difference across the parallel combination: \(V_p = I_T \times R_p = 2.4\text{ A} \times 3.0\ \Omega = 7.2\text{ V}\). 5. Calculate the current in the \(12.0\ \Omega\) resistor: \(I = V_p / 12.0\ \Omega = 7.2\text{ V} / 12.0\ \Omega = 0.6\text{ A}\).
評分準則
Award 1 mark for the correct answer A. Method involves calculating parallel resistance (3.0 ohms), total resistance (5.0 ohms), total current (2.4 A), parallel voltage (7.2 V), and finally the current through the 12.0 ohm resistor (0.6 A).
題目 26 · 選擇題
1 分
When magnesium ribbon is placed in a solution of copper(II) sulfate, a displacement reaction occurs: \(\text{Mg (s)} + \text{Cu}^{2+}\text{ (aq)} \rightarrow \text{Mg}^{2+}\text{ (aq)} + \text{Cu (s)}\). Which statement correctly describes this reaction in terms of electron transfer?
A.Copper(II) ions are reduced because they gain electrons.
B.Copper(II) ions are oxidized because they lose electrons.
C.Magnesium atoms are reduced because they lose electrons.
D.Magnesium atoms are oxidized because they gain electrons.
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解題
In this redox reaction, magnesium atoms (\(\text{Mg}\)) lose two electrons to form magnesium ions (\(\text{Mg}^{2+}\)), which is oxidation (loss of electrons). Copper(II) ions (\(\text{Cu}^{2+}\)) gain two electrons to form copper atoms (\(\text{Cu}\)), which is reduction (gain of electrons).
評分準則
Award 1 mark for identifying that copper(II) ions are reduced because they gain electrons (A).
題目 27 · 選擇題
1 分
The rate of an enzyme-controlled reaction is measured at different temperatures. Which statement explains the decrease in the rate of reaction at temperatures above the optimum temperature?
A.The kinetic energy of the enzyme and substrate molecules decreases, reducing collision frequency.
B.The increased thermal energy breaks bonds maintaining the enzyme's three-dimensional shape, so the substrate no longer fits the active site.
C.The activation energy of the reaction increases, making it harder for the reaction to occur.
D.The substrate molecules denature and change shape, so they can no longer fit the active site.
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解題
At temperatures above the optimum, the increased thermal energy increases the vibration of the atoms in the enzyme molecule. This breaks the bonds that maintain its specific three-dimensional active site shape. The enzyme becomes denatured, meaning the substrate is no longer complementary to the active site, and the rate of reaction decreases rapidly.
評分準則
Award 1 mark for the correct explanation that thermal energy breaks bonds maintaining the active site shape, leading to loss of fit (B).
題目 28 · 選擇題
1 分
An electromagnetic wave travels through a vacuum. It has a wavelength of \(1.5 \times 10^{-2}\text{ m}\).
Which row correctly identifies the frequency of this wave and the region of the electromagnetic spectrum to which it belongs?
(The speed of electromagnetic waves in a vacuum is \(3.0 \times 10^8\text{ m/s}\).)
A wavelength of \(1.5 \times 10^{-2}\text{ m}\) (or \(1.5\text{ cm}\)) falls within the range of \(1\text{ mm}\) to \(1\text{ m}\), which is the microwave region of the electromagnetic spectrum. Thus, option A is correct.
評分準則
1 mark: Correctly calculates the wave frequency as \(2.0 \times 10^{10}\text{ Hz}\) and identifies the region as microwave.
題目 29 · 選擇題
1 分
Concentrated aqueous sodium chloride is electrolysed using inert carbon electrodes.
What are the major products formed at the electrodes, and how does the pH of the remaining electrolyte change during the process?
A.Cathode product: hydrogen; Anode product: chlorine; pH of electrolyte: increases
B.Cathode product: sodium; Anode product: chlorine; pH of electrolyte: decreases
C.Cathode product: hydrogen; Anode product: oxygen; pH of electrolyte: increases
During the electrolysis of concentrated aqueous sodium chloride: - At the cathode (negative electrode), hydrogen ions (\(\text{H}^+\)) from water are discharged in preference to sodium ions (\(\text{Na}^+\)) because hydrogen is less reactive. This produces hydrogen gas (\(\text{H}_2\)). - At the anode (positive electrode), chloride ions (\(\text{Cl}^-\)) from the concentrated salt are discharged in preference to hydroxide ions (\(\text{OH}^-\)). This produces chlorine gas (\(\text{Cl}_2\)). - As \(\text{H}^+\) and \(\text{Cl}^-\) ions are discharged, \(\text{Na}^+\) and \(\text{OH}^-\) ions remain in the solution, forming sodium hydroxide (\(\text{NaOH}\)), which is alkaline. Therefore, the pH of the electrolyte increases.
評分準則
1 mark: Correctly identifies the cathode product as hydrogen, the anode product as chlorine, and that the pH of the electrolyte increases.
題目 30 · 選擇題
1 分
Why does the rate of an enzyme-catalysed reaction increase when the temperature is raised from \(20\text{ }^\circ\text{C}\) to \(35\text{ }^\circ\text{C}\)?
A.The kinetic energy of the substrate and enzyme molecules increases, resulting in more frequent collisions between active sites and substrate molecules.
B.The enzyme molecules expand, causing their active sites to become larger and hold more substrate molecules.
C.The activation energy required for the reaction is lowered because of the increase in temperature.
D.The enzyme molecules denature, which allows the substrate molecules to bind to any part of the enzyme surface.
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解題
Increasing the temperature from \(20\text{ }^\circ\text{C}\) to \(35\text{ }^\circ\text{C}\) (which is near the optimum temperature for many enzymes) increases the kinetic energy of both the enzyme and substrate molecules. As a result, they move faster and collide more frequently, leading to a higher rate of successful collisions between the substrate and the enzyme's active site. Option B is incorrect because active sites do not expand to hold more substrates. Option C is incorrect because temperature changes do not alter the activation energy of the pathway. Option D is incorrect because denaturation occurs at higher temperatures (typically above \(40\text{ }^\circ\text{C}\)- \(45\text{ }^\circ\text{C}\)) and would decrease the reaction rate.
評分準則
1 mark: Correctly identifies that the increased kinetic energy leads to more frequent collisions between active sites and substrate molecules.
題目 31 · 選擇題
1 分
A ray of light is incident on the flat surface of a glass block at an angle of incidence of \(48^\circ\). The refractive index of the glass is \(1.50\). What is the approximate angle of refraction inside the glass? (Take \(\sin 48^\circ = 0.74\))
A.\(20^\circ\)
B.\(30^\circ\)
C.\(32^\circ\)
D.\(72^\circ\)
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解題
The relationship between the angle of incidence, \(i\), the angle of refraction, \(r\), and the refractive index, \(n\), is given by the formula: \(n = \frac{\sin i}{\sin r}\). Rearranging to solve for \(\sin r\) gives \(\sin r = \frac{\sin i}{n}\). Substituting the values, we get \(\sin r = \frac{0.74}{1.50} \approx 0.493\). Since \(\sin 30^\circ = 0.50\), the angle of refraction is approximately \(30^\circ\).
評分準則
C1 for identifying the correct refractive index formula. A1 for calculating the correct angle of refraction.
題目 32 · 選擇題
1 分
A student investigates the rate of reaction between marble chips (calcium carbonate) and dilute hydrochloric acid: \(\text{CaCO}_3(\text{s}) + 2\text{HCl}(\text{aq}) \rightarrow \text{CaCl}_2(\text{aq}) + \text{H}_2\text{O}(\text{l}) + \text{CO}_2(\text{g})\). Which change to the reaction conditions increases the frequency of collisions between reacting particles, but does NOT increase the average kinetic energy of the particles?
A.adding a catalyst
B.increasing the concentration of the acid
C.increasing the temperature of the acid
D.using larger pieces of marble chips
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解題
Increasing the concentration increases the number of reacting particles per unit volume, which increases the frequency of collisions. However, it does not alter the average kinetic energy of the particles, as that is only increased by raising the temperature.
評分準則
A1 for selecting the correct change that increases collision frequency without affecting kinetic energy.
題目 33 · 選擇題
1 分
An enzyme-catalysed reaction is carried out at four different temperatures: \(20^\circ\text{C}\), \(40^\circ\text{C}\), \(60^\circ\text{C}\), and \(80^\circ\text{C}\). The rate of reaction is highest at \(40^\circ\text{C}\). At \(80^\circ\text{C}\), the reaction does not occur at all because the enzyme has denatured. Which statement correctly describes the enzyme molecules at \(40^\circ\text{C}\) and \(80^\circ\text{C}\)?
A.At \(40^\circ\text{C}\), the active site has a complementary shape to the substrate. At \(80^\circ\text{C}\), the shape of the active site has changed.
B.At \(40^\circ\text{C}\), the enzyme molecules have the highest kinetic energy. At \(80^\circ\text{C}\), the enzyme molecules have no kinetic energy.
C.At \(40^\circ\text{C}\), the active site has a complementary shape to the substrate. At \(80^\circ\text{C}\), the substrate has been denatured.
D.At \(40^\circ\text{C}\), the enzyme has denatured. At \(80^\circ\text{C}\), the active site is complementary to the substrate.
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解題
At \(40^\circ\text{C}\) (the optimum temperature), the active site of the enzyme has a complementary shape to the substrate, allowing rapid binding. At \(80^\circ\text{C}\), the enzyme (which is a protein) is denatured, meaning the shape of its active site has changed permanently and can no longer fit the substrate.
評分準則
A1 for identifying both the complementary nature of the active site at optimum temperature and denaturation at high temperature.
題目 34 · 選擇題
1 分
Why does the rate of an enzyme-catalysed reaction decrease rapidly at temperatures above the optimum temperature?
A.The kinetic energy of the enzyme and substrate molecules decreases, resulting in fewer collisions.
B.The active site of the enzyme changes shape permanently, so the substrate is no longer complementary.
C.The activation energy of the reaction increases, meaning more energy is required for the reaction to occur.
D.The substrate molecules denature at high temperatures, preventing them from fitting into the active site.
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解題
At temperatures above the optimum, the excessive thermal energy causes the bonds holding the enzyme's specific three-dimensional structure together to break. This permanently changes the shape of the active site (denaturation). Consequently, the substrate is no longer complementary and cannot fit into the active site, causing the reaction rate to fall rapidly.
評分準則
1 mark for selecting the correct explanation of denaturation of the enzyme's active site (B).
題目 35 · 選擇題
1 分
The methods used to extract three metals, \(X\), \(Y\) and \(Z\), from their ores are described. Metal \(X\) is extracted by heating its oxide with carbon. Metal \(Y\) is found native (as the uncombined element) in the Earth's crust. Metal \(Z\) is extracted from its molten ore by electrolysis. What is the order of reactivity of the metals, from most reactive to least reactive?
A.\(X \rightarrow Y \rightarrow Z\)
B.\(Y \rightarrow X \rightarrow Z\)
C.\(Z \rightarrow X \rightarrow Y\)
D.\(Z \rightarrow Y \rightarrow X\)
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解題
Metals high in the reactivity series (more reactive than carbon) form very stable compounds and can only be extracted by electrolysis, meaning metal \(Z\) is the most reactive. Metals below carbon can be extracted by reduction with carbon, so metal \(X\) is of medium reactivity. Extremely unreactive metals are found native as elements, so metal \(Y\) is the least reactive. The correct order from most to least reactive is \(Z \rightarrow X \rightarrow Y\).
評分準則
1 mark for correctly identifying the reactivity order as Z (most reactive, electrolysis), X (medium reactivity, carbon reduction), and Y (least reactive, native) (C).
題目 36 · 選擇題
1 分
A radio transmitter broadcasts a signal at a frequency of \(1.5\text{ MHz}\). The speed of radio waves in air is \(3.0 \times 10^8\text{ m/s}\). What is the wavelength of the radio waves?
A.\(0.005\text{ m}\)
B.\(2.0\text{ m}\)
C.\(200\text{ m}\)
D.\(4.5 \times 10^{14}\text{ m}\)
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解題
Using the wave equation: \(v = f \lambda\), where \(v = 3.0 \times 10^8\text{ m/s}\) and \(f = 1.5\text{ MHz} = 1.5 \times 10^6\text{ Hz}\). Rearranging for wavelength gives \(\lambda = \frac{v}{f} = \frac{3.0 \times 10^8\text{ m/s}}{1.5 \times 10^6\text{ Hz}} = 200\text{ m}\).
評分準則
1 mark for converting the frequency to Hz and calculating the wavelength using the wave equation (C).
題目 37 · 選擇題
1 分
A car of mass 800 kg is travelling at a constant speed of 20 m/s. The driver applies the brakes, bringing the car to rest with a uniform deceleration in a time of 4.0 s. What is the average braking force acting on the car?
A.160 N
B.1600 N
C.4000 N
D.16000 N
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解題
First, calculate the deceleration: a = (v - u) / t = (0 - 20) / 4.0 = -5.0 m/s^2. Then, use Newton's second law: F = m * a = 800 kg * 5.0 m/s^2 = 4000 N.
評分準則
1 mark for the correct calculation leading to 4000 N (Option C).
題目 38 · 選擇題
1 分
Which row correctly describes how increasing the concentration of a reactant solution affects the collisions between reacting particles?
A.collision frequency increases; proportion of collisions with energy greater than or equal to activation energy increases
B.collision frequency increases; proportion of collisions with energy greater than or equal to activation energy remains unchanged
C.collision frequency remains unchanged; proportion of collisions with energy greater than or equal to activation energy increases
D.collision frequency remains unchanged; proportion of collisions with energy greater than or equal to activation energy remains unchanged
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解題
Increasing concentration increases the number of reactant particles per unit volume, which increases collision frequency. It does not change temperature, so the proportion of collisions with energy greater than or equal to the activation energy remains unchanged.
評分準則
1 mark for identifying both effects correctly (Option B).
題目 39 · 選擇題
1 分
Which statement describes the effect on an enzyme when it is heated to a temperature far above its optimum temperature?
A.The kinetic energy of the enzyme molecules decreases, which prevents them from colliding with substrate molecules.
B.The shape of the active site changes, preventing substrate molecules from binding to it.
C.The chemical bonds within the substrate molecules are strengthened, preventing them from being broken down.
D.The enzyme molecule is fully hydrolysed into its constituent amino acids.
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解題
Heating an enzyme far above its optimum temperature causes it to denature. This changes the specific three-dimensional shape of its active site, preventing substrate molecules from binding.
評分準則
1 mark for identifying that the active site shape changes and prevents binding (Option B).
題目 40 · 選擇題
1 分
Aqueous copper(II) sulfate is electrolysed using inert carbon electrodes. Which row shows the correct observations made at each electrode?
A.Anode: bubbles of a colourless gas | Cathode: a pink-brown solid is deposited
B.Anode: bubbles of a colourless gas | Cathode: bubbles of a colourless gas
C.Anode: a pink-brown solid is deposited | Cathode: bubbles of a colourless gas
D.Anode: bubbles of a green gas | Cathode: a pink-brown solid is deposited
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解題
During the electrolysis of aqueous copper(II) sulfate using inert carbon electrodes: At the anode (positive electrode), hydroxide ions from water are preferentially discharged to produce oxygen gas, which is observed as bubbles of a colourless gas. At the cathode (negative electrode), copper(II) ions are preferentially discharged to form copper metal, which is observed as a pink-brown solid deposited on the electrode. Therefore, option A is correct.
評分準則
1 mark for identifying the correct observations at the anode (bubbles of a colourless gas) and cathode (a pink-brown solid deposited).
Paper 31 (Theory Core)
Structured theory questions based on the Core syllabus. Answer all questions.
9 題目 · 79.92 分
題目 1 · structured
8.88 分
A student is observing water waves on the surface of a pond.
(a) Define the term *wavelength*. [1]
(b) The water waves travel a distance of \(12\text{ m}\) in a time of \(4.0\text{ s}\). Calculate the speed of the water waves. State the unit. [2]
(c) The frequency of these water waves is \(1.5\text{ Hz}\). Show that the wavelength of these waves is \(2.0\text{ m}\). [3]
(d) Water waves are transverse waves. Sound waves are longitudinal waves. State one difference between a transverse wave and a longitudinal wave in terms of the direction of vibration relative to the direction of energy transfer. [1]
(e) Electromagnetic waves also travel as transverse waves. (i) State one hazard of exposure to ultraviolet radiation. [1] (ii) State one common use of infrared radiation. [0.88]
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解題
(a) The wavelength is defined as the distance between two successive crests or troughs (or any two identical points in phase). (b) Using the speed formula: \(\text{speed} = \frac{\text{distance}}{\text{time}}\) \(\text{speed} = \frac{12\text{ m}}{4.0\text{ s}} = 3.0\text{ m/s}\). (c) Using the wave equation: \(\text{speed} = \text{frequency} \times \text{wavelength}\) \(\text{wavelength} = \frac{\text{speed}}{\text{frequency}}\) \(\text{wavelength} = \frac{3.0\text{ m/s}}{1.5\text{ Hz}} = 2.0\text{ m}\). (d) Transverse waves vibrate perpendicular (at right angles) to the direction of energy travel, while longitudinal waves vibrate parallel to the direction of energy travel. (e) (i) Hazards of ultraviolet (UV) radiation include damage to surface cells, sunburn, skin cancer, or eye damage. (ii) Common uses of infrared (IR) include remote controls, electrical appliances, grills, thermal imaging, or optical fibers.
評分準則
- (a) [1 mark] for stating that wavelength is the distance between two adjacent/consecutive crests (or troughs) / distance over which the wave shape repeats. - (b) [1 mark] for correct calculation: \(3.0\), [1 mark] for correct unit: \(\text{m/s}\) (or \(\text{m s}^{-1}\)). - (c) [1 mark] for recalling formula \(v = f \lambda\) or \(\lambda = v/f\); [1 mark] for substitution of values \(\lambda = 3.0 / 1.5\); [1 mark] for obtaining \(2.0\text{ m}\). - (d) [1 mark] for stating that vibrations in transverse waves are perpendicular to energy transfer, while in longitudinal waves they are parallel. - (e)(i) [1 mark] for any one valid hazard of UV (e.g., sunburn, skin cancer, blindness, cell damage). - (e)(ii) [0.88 marks] for any one valid use of IR (e.g., TV remote control, night-vision/thermal imaging, radiant heaters, cooking, optical fibres).
題目 2 · structured
8.88 分
A student investigates the rate of reaction between calcium carbonate chips and dilute hydrochloric acid.
(a) Write the word equation for this reaction. [2]
(b) The student repeats the experiment using the same mass of calcium carbonate and same volume of acid, but uses a higher temperature of the acid. (i) Describe the effect of increasing the temperature on the rate of this reaction. [1] (ii) Explain this effect in terms of the collision of particles. [2]
(c) State and explain the effect of using finely powdered calcium carbonate instead of large chips on the rate of reaction. [2]
(d) Describe a chemical test to confirm that the gas produced during this reaction is carbon dioxide. State the result of a positive test. [1.88]
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解題
(a) The reactants are calcium carbonate and hydrochloric acid. The products are calcium chloride, water, and carbon dioxide. \(\text{calcium carbonate} + \text{hydrochloric acid} \rightarrow \text{calcium chloride} + \text{water} + \text{carbon dioxide}\) (b) (i) Increasing the temperature increases the rate of reaction. (ii) At higher temperatures, particles have more kinetic energy and move faster. This leads to a higher frequency of collisions (more collisions per second). (c) Finely powdered calcium carbonate has a larger surface area than large chips. A larger surface area increases the rate of reaction because more particles are exposed, leading to more frequent collisions. (d) The gas is bubbled through limewater (aqueous calcium hydroxide). If the gas is carbon dioxide, the limewater turns cloudy, milky, or forms a white precipitate.
評分準則
- (a) [1 mark] for correct reactants: calcium carbonate + hydrochloric acid; [1 mark] for correct products: calcium chloride + water + carbon dioxide. - (b)(i) [1 mark] for stating that the rate of reaction increases / reaction becomes faster. - (b)(ii) [1 mark] for stating that particles move faster / have more kinetic energy; [1 mark] for stating that they collide more frequently / more collisions per unit time. - (c) [1 mark] for stating that rate of reaction increases; [1 mark] for stating that powder has a larger surface area (resulting in more frequent collisions). - (d) [1 mark] for bubble gas through limewater / add to limewater; [0.88 marks] for stating that it turns cloudy / milky / white precipitate forms.
題目 3 · structured
8.88 分
Enzymes are vital biological catalysts that speed up chemical reactions inside living organisms.
(a) State the chemical substance group that enzymes belong to. [1]
(b) Name the four chemical elements present in all enzyme molecules. [2.88]
(c) An experiment is carried out to investigate the effect of temperature on the rate of an enzyme-controlled reaction. The rate of reaction increases from \(10\text{ }^{\circ}\text{C}\) up to an optimum of \(40\text{ }^{\circ}\text{C}\), then drops sharply to zero by \(65\text{ }^{\circ}\text{C}\). (i) State the optimum temperature for this enzyme. [1] (ii) Explain why the rate of reaction decreases rapidly at temperatures above \(40\text{ }^{\circ}\text{C}\) and stops completely by \(65\text{ }^{\circ}\text{C}\). Use the term *denatured* in your explanation. [3]
(d) State the effect of an extremely high or extremely low pH on the activity of most enzymes. [1]
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解題
(a) Enzymes are proteins. (b) Proteins (and therefore enzymes) are made of the elements carbon, hydrogen, oxygen, and nitrogen. (c) (i) The optimum temperature is the temperature at which the rate of reaction is highest, which is \(40\text{ }^{\circ}\text{C}\). (ii) Above the optimum temperature, the high thermal energy causes the chemical bonds holding the enzyme's three-dimensional structure together to break. This changes the shape of the active site. The substrate molecule can no longer fit into the active site. The enzyme is denatured, and the reaction stops. (d) Extremely high or low pH values (away from the optimum pH) will also denature the enzyme, reducing its activity or stopping it completely.
評分準則
- (a) [1 mark] for protein(s). - (b) [2.88 marks] for naming: carbon, hydrogen, oxygen, and nitrogen (allow 0.72 marks per correct element up to 4; accept C, H, O, N). - (c)(i) [1 mark] for \(40\text{ }^{\circ}\text{C}\) (accept 40). - (c)(ii) [1 mark] for mentioning that the active site changes shape; [1 mark] for stating that the substrate can no longer fit / bind to the active site; [1 mark] for stating that the enzyme is denatured. - (d) [1 mark] for stating that the activity decreases / the enzyme denatures / activity stops.
題目 4 · structured
8.88 分
Amylase is an enzyme that catalyses the breakdown of starch into simple sugars.
(a) Define the term *enzyme*. [2]
(b) A student investigates the effect of temperature on the rate of reaction of amylase. Explain why the rate of reaction increases as the temperature is raised from \(10^\circ\text{C}\) to \(35^\circ\text{C}\). [2.88]
(c) The investigation is repeated at \(80^\circ\text{C}\). At this temperature, the reaction stops completely. State what has happened to the active site of the amylase enzyme at this temperature, and explain how this affects its function. [4]
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解題
(a) An enzyme is defined as a protein that functions as a biological catalyst. Both parts (protein and biological catalyst) are key to the core definition.
(b) Raising the temperature from \(10^\circ\text{C}\) to \(35^\circ\text{C}\) gives the enzyme and starch molecules more kinetic energy. They move faster and collide more frequently, increasing the probability of successful collisions per unit time, thereby increasing the rate of reaction.
(c) At high temperatures such as \(80^\circ\text{C}\), the thermal energy breaks bonds holding the enzyme's three-dimensional shape together. This changes the shape of the active site (denaturation). Because the shape of the active site is no longer complementary to the starch substrate, the substrate cannot bind to it, and the reaction stops.
(b) [Total: 2.88 marks] - (molecules have) more kinetic energy [1] - move faster / more frequent collisions [1] - more successful collisions / enzyme-substrate complexes formed per unit time [0.88]
(c) [Total: 4 marks] - enzyme is denatured [1] - active site changes shape [1] - substrate / starch no longer fits into the active site [1] - no enzyme-substrate complexes can form / no reaction can occur [1]
題目 5 · structured
8.88 分
This question is about metals and their reactivity.
(a) A student tests four metals, \(W\), \(X\), \(Y\), and \(Z\), by reacting each metal with solutions containing the nitrates of the other metals. The observations are summarized below: - Metal \(W\) reacts with solutions of \(X^{2+}\) and \(Z^{2+}\), but does not react with \(Y^{2+}\). - Metal \(Z\) does not react with any of the solutions.
Deduce the order of reactivity of these four metals, from most reactive to least reactive. [2.88]
(b) Iron is extracted from its ore in a Blast Furnace.
(i) State the name of the main iron ore used in this process. [1]
(ii) Carbon monoxide (\(\text{CO}\)) reacts with iron(III) oxide (\(\text{Fe}_2\text{O}_3\)) to produce iron and carbon dioxide. Complete the balanced chemical equation for this reaction: \(\text{Fe}_2\text{O}_3 + \dots \text{CO} \rightarrow \dots \text{Fe} + \dots \text{CO}_2\) [2]
(iii) State which substance is reduced in this reaction, and explain your choice in terms of oxygen transfer. [3]
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解題
(a) To determine the reactivity order: - \(W\) displacement of \(X\) and \(Z\) means \(W > X\) and \(W > Z\). - \(W\) does not displace \(Y\), meaning \(Y > W\). - \(Z\) is completely unreactive with the other metal ions, making it the least reactive. Thus, the order of reactivity from most to least is: \(Y, W, X, Z\).
(b) (i) The primary ore from which iron is extracted is hematite (containing mainly iron(III) oxide). (ii) Balancing the equation: One mole of \(\text{Fe}_2\text{O}_3\) reacts with three moles of \(\text{CO}\) to yield two moles of iron and three moles of carbon dioxide. Thus, the coefficients are: \(1\) (for \(\text{Fe}_2\text{O}_3\), blank/implied), \(3\) (for \(\text{CO}\)), \(2\) (for \(\text{Fe}\)), and \(3\) (for \(\text{CO}_2\)). (iii) Iron(III) oxide (\(\text{Fe}_2\text{O}_3\)) is reduced. Reduction is defined as the loss of oxygen; the iron(III) oxide loses its oxygen atoms to carbon monoxide, forming elemental iron.
評分準則
(a) [Total: 2.88 marks] - Correct order: \(Y, W, X, Z\) [2.88] - Allow [1.88] if reversed order (least to most reactive) is given: \(Z, X, W, Y\). - Allow [1.00] if \(Y\) is correctly identified as most reactive and \(Z\) as least reactive but the middle two are incorrect.
(ii) [Total: 2 marks] - Correct balance of carbon-containing species: \(3\text{CO}\) and \(3\text{CO}_2\) [1] - Correct balance of iron: \(2\text{Fe}\) [1]
(iii) [Total: 3 marks] - iron(III) oxide / \(\text{Fe}_2\text{O}_3\) (accept: iron oxide) [1] - is reduced because it loses oxygen [2] (award [1] for general statement that "reduction is the loss of oxygen" if not explicitly linked to the chemical species)
題目 6 · structured
8.88 分
Waves can be used to transfer energy and information.
(a) A water wave has a frequency of \(5.0\text{ Hz}\) and a wavelength of \(0.12\text{ m}\).
(i) Calculate the speed of this water wave. State the formula used, show your working and state the unit. [3.88]
(ii) Define the term *frequency* of a wave. [1]
(b) Draw lines matching each region of the electromagnetic spectrum to its correct common use. [4]
* **Region of Spectrum:** * Microwaves * Infrared * X-rays * Gamma rays * **Common Use:** * Intruder alarms and remote controllers * Killing cancerous cells and sterilising medical equipment * Satellite television and mobile phone communications * Security scanners at airports and medical imaging
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解題
(a) (i) The formula for wave speed is \(v = f \lambda\), where \(v\) is speed, \(f\) is frequency, and \(\lambda\) is wavelength. Substituting the values: \(v = 5.0\text{ Hz} \times 0.12\text{ m} = 0.60\text{ m/s}\).
(ii) Frequency is defined as the number of complete waves (or cycles) that pass a fixed point per unit time (usually per second).
(b) Matchings based on standard syllabus uses: - Microwaves are used for satellite television and mobile communications because they can pass through the atmosphere. - Infrared is used in remote controllers and intruder alarms (passive infrared sensors). - X-rays are highly penetrating and absorbed differently by materials of different densities, making them ideal for security scanners and bone imaging. - Gamma rays have very high energy, useful for sterilising equipment and radiotherapy (killing cancer cells).
評分準則
(a) (i) [Total: 3.88 marks] - State formula: speed = frequency \(\times\) wavelength (or \(v = f \lambda\)) [1] - Correct substitution: \(5.0 \times 0.12\) [1] - Correct calculation: \(0.6\) or \(0.60\) [1] - Correct unit: \(\text{m/s}\) (or metres per second) [0.88]
(ii) [Total: 1 mark] - number of waves passing a point per second / per unit time [1]
(b) [Total: 4 marks] - Microwaves connected to "Satellite television and mobile phone communications" [1] - Infrared connected to "Intruder alarms and remote controllers" [1] - X-rays connected to "Security scanners at airports and medical imaging" [1] - Gamma rays connected to "Killing cancerous cells and sterilising medical equipment" [1]
題目 7 · structured
8.88 分
A student investigates osmosis using potato cylinders.
(a) Define osmosis in terms of water molecules. [2]
(b) A potato cylinder with an initial mass of 5.0 g is placed in a concentrated sucrose solution. (i) Predict the change, if any, in the mass of the potato cylinder after 2 hours. [1] (ii) Explain your prediction in terms of water potential and the movement of water molecules. [2.88]
(c) State two variables, other than the concentration and volume of the sucrose solution, that must be kept constant to ensure a fair test. [2]
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解題
(a) Osmosis is the net movement of water molecules from a region of higher water potential (dilute solution) to a region of lower water potential (concentrated solution), through a partially permeable membrane.
(b) (i) The mass of the potato cylinder will decrease. (ii) The water potential inside the potato cells is higher than the water potential of the concentrated sucrose solution outside. Water molecules move out of the cells down the water potential gradient by osmosis, causing a reduction in mass.
(c) Any two from: temperature of the solution, surface area / dimensions of the potato cylinders, variety / type of potato, or the time the cylinders are left in the solution.
評分準則
(a) - net movement of water molecules from a region of higher water potential to a region of lower water potential [1 mark] - through a partially permeable membrane [1 mark]
(b)(i) - mass decreases / gets lighter [1 mark]
(b)(ii) - water potential is higher inside potato cells than in sucrose solution [1 mark] - water moves out of the potato cells by osmosis [1 mark] - loss of water causes decrease in mass [0.88 marks]
(c) - any two from: temperature of solution, surface area/dimensions of potato, type/source of potato, immersion time [2 marks, 1 mark for each correct variable]
(a) State which reactant is reduced in this reaction. Explain your answer in terms of oxygen transfer. [2]
(b) Carbon dioxide is produced in this reaction. Describe a chemical test for carbon dioxide gas, including the observation for a positive result. [2]
(c) State why carbon is able to react with copper(II) oxide in terms of the reactivity series of metals. [2.88]
(d) Describe one safety precaution that should be taken during this experiment, other than wearing safety goggles, and explain why it is necessary. [2]
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解題
(a) Copper(II) oxide is reduced because it loses oxygen during the reaction.
(b) Bubble the gas through limewater. The limewater will turn cloudy or milky if carbon dioxide is present.
(c) Carbon is more reactive than copper in the reactivity series. Therefore, carbon can displace copper from its oxide.
(d) Conduct the heating in a well-ventilated area or a fume cupboard because toxic gases like carbon monoxide can be produced, OR use tongs to handle the hot test tubes to prevent burns.
評分準則
(a) - copper(II) oxide [1 mark] - loss of oxygen / oxygen is removed [1 mark]
(b) - test: bubble the gas through limewater [1 mark] - observation: turns cloudy / milky / white precipitate forms [1 mark]
(c) - carbon is more reactive than copper [1.88 marks] - carbon displaces copper from copper oxide [1 mark]
(d) - valid safety precaution (e.g., use of fume cupboard / handle hot objects with tongs or heatproof gloves) [1 mark] - correct scientific reason linked to precaution (e.g., toxic gases produced / prevent burns) [1 mark]
題目 9 · structured
8.88 分
A student uses a ripple tank to study water waves.
(a) Water waves are transverse waves. State the difference between transverse waves and longitudinal waves in terms of their direction of vibration relative to the direction of energy transfer. [2]
(b) The water waves have a frequency of \(5.0\text{ Hz}\) and a wavelength of \(0.040\text{ m}\). (i) State the equation that links wave speed, frequency, and wavelength. [1] (ii) Calculate the speed of the water waves. Show your working and state the unit. [2.88]
(c) The waves then travel from deep water into shallow water. The speed of the waves decreases, but the frequency remains constant. State and explain the effect of this change on the wavelength of the waves. [3]
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解題
(a) In transverse waves, the vibrations are perpendicular to the direction of energy transfer, whereas in longitudinal waves, the vibrations are parallel to the direction of energy transfer.
(c) The wavelength decreases. Since wave speed \(v = f \lambda\), if the frequency \(f\) remains constant and the speed \(v\) decreases, the wavelength \(\lambda\) must also decrease to maintain the relationship.
評分準則
(a) - transverse: vibrations are perpendicular to the direction of energy transfer [1 mark] - longitudinal: vibrations are parallel to the direction of energy transfer [1 mark]
(c) - wavelength decreases [1 mark] - wavelength is directly proportional to speed (when frequency is constant) / formula reference [1 mark] - since frequency \(f\) is constant, a smaller speed \(v\) results in a smaller wavelength \(\lambda\) [1 mark]
Paper 41 (Theory Extended)
Structured theory questions based on the Extended syllabus. Answer all questions.
9 題目 · 79.92 分
題目 1 · structured
8.88 分
A sound wave is a type of mechanical wave. (a) State the difference between a transverse wave and a longitudinal wave in terms of the direction of vibration of particles relative to the direction of wave propagation. [3] (b) A sound wave traveling through water has a frequency of \(2500\text{ Hz}\). The speed of sound in water is \(1500\text{ m/s}\). (i) Calculate the wavelength of this sound wave. Show your working and state the unit. [3] (ii) State and explain what happens to the speed of the sound wave when it passes from water into air. [2]
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解題
(a) In a transverse wave, the vibration of particles is perpendicular (at right angles) to the direction of wave propagation. In a longitudinal wave, the vibration of particles is parallel to the direction of wave propagation. (b)(i) \(v = f \lambda \Rightarrow \lambda = \frac{v}{f} = \frac{1500}{2500} = 0.60\text{ m}\). (ii) The speed decreases because sound travels slower in gases (air) than in liquids (water) as the particles are much further apart in gases, leading to less efficient energy transmission.
評分準則
(a) Transverse wave: vibration is perpendicular to the direction of energy transfer / wave propagation. [1] Longitudinal wave: vibration is parallel to the direction of energy transfer / wave propagation. [1] Correct comparison of both terms. [1] (b)(i) Recall of formula: \(v = f\lambda\) or \(\lambda = \frac{v}{f}\) [1] Correct substitution and calculation: 0.6 / 0.60 [1] Correct unit: m / metres [1] (b)(ii) Speed decreases [1] Air is less dense / particles are further apart than in water, meaning fewer collisions to transmit the sound wave [1]
題目 2 · structured
8.88 分
A student carries out the electrolysis of aqueous copper(II) sulfate, \(\text{CuSO}_4\text{(aq)}\), using inert carbon electrodes. (a) Name the product formed at the anode (positive electrode) and state the observation that confirms this product. [2] (b) At the cathode (negative electrode), copper ions are reduced to copper metal. (i) Write the ionic half-equation for this reduction reaction. [2] (ii) State the observation at the cathode. [1] (c) During the electrolysis, the color of the electrolyte changes. (i) Describe the color change of the solution and explain why this change occurs. [2] (ii) State what happens to the pH of the solution as the electrolysis continues, and explain why. [2]
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解題
(a) Product: Oxygen. Observation: Bubbles / effervescence. (b)(i) \(\text{Cu}^{2+} + 2\text{e}^- \rightarrow \text{Cu}\). (ii) Pink/brown solid deposit. (c)(i) The blue color fades/becomes colorless because copper(II) ions (\(\text{Cu}^{2+}\)) are removed from the solution. (ii) The pH decreases (becomes acidic) because hydrogen ions (\(\text{H}^+\)) and sulfate ions (\(\text{SO}_4^{2-}\)) remain in solution, forming sulfuric acid.
評分準則
(a) Product: Oxygen [1] (Accept \(\text{O}_2\)). Observation: Bubbles / effervescence / colorless gas [1]. (b)(i) Correct reactant and product: \(\text{Cu}^{2+}\) and \(\text{Cu}\) [1], balanced with \(2\text{e}^-\)[1]. (b)(ii) Pink/brown solid [1] (Reject: copper-colored, red-brown). (c)(i) Blue color fades / becomes colorless [1]. Copper(II) ions are discharged / concentration of copper(II) ions decreases [1]. (c)(ii) pH decreases [1]. Hydrogen ions (\(\text{H}^+\)) remain in solution / sulfuric acid is formed [1].
題目 3 · structured
8.88 分
Enzymes are vital proteins that act as biological catalysts in living organisms. (a) Explain what is meant by the term biological catalyst. [2] (b) Describe how an enzyme catalyzes the breakdown of a substrate molecule, referring to the 'lock and key' hypothesis. [3] (c) An experiment was carried out to investigate the effect of temperature on the rate of an enzyme-controlled reaction. (i) Describe and explain the effect on the rate of reaction as the temperature is increased from \(10\ ^\circ\text{C}\) to the enzyme's optimum temperature of \(37\ ^\circ\text{C}\). [2] (ii) Explain why the rate of reaction drops rapidly to zero when the temperature is increased to \(60\ ^\circ\text{C}\). [2]
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解題
(a) A biological catalyst is a protein that increases the rate of chemical reactions in living cells without being consumed. (b) The substrate has a shape complementary to the enzyme's active site. They bind to form an enzyme-substrate complex. The reaction occurs and products are released, leaving the active site unchanged. (c)(i) The rate increases because higher temperature gives molecules more kinetic energy, leading to more frequent successful collisions between enzyme and substrate. (ii) At \(60\ ^\circ\text{C}\), the enzyme is denatured. The active site changes shape, meaning the substrate is no longer complementary and cannot bind.
評分準則
(a) Increases rate of a chemical reaction [1]. Is a protein / is not used up in the reaction [1]. (b) Substrate has complementary shape to the active site [1]. Substrate binds to active site / forms enzyme-substrate complex [1]. Products released and active site is unchanged [1]. (c)(i) Rate increases [1]. Due to more kinetic energy resulting in more frequent collisions [1]. (c)(ii) Enzyme is denatured / active site changes shape [1]. Substrate no longer fits / cannot bind [1] (Reject: enzyme is killed).
題目 4 · structured
8.88 分
(a) A marine researcher uses a sonar device to detect fish under water. The sonar device emits ultrasound of frequency \(40\text{ kHz}\).
(i) Explain what is meant by a longitudinal wave, referring to the direction of vibration of the particles relative to the direction of energy transfer. [2]
(ii) The speed of sound in water is \(1500\text{ m/s}\). Calculate the wavelength of this ultrasound wave in water. Show your working and state the unit. [3]
(b) The researcher also uses a radio transmitter operating at a frequency of \(3.0 \times 10^8\text{ Hz}\) to send data back to shore through air.
(i) State the value of the speed of radio waves in air. [1]
(ii) Explain, in terms of wave types, why radio waves can travel through a vacuum but ultrasound waves cannot. [2.88]
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解題
(a)(i) A longitudinal wave is defined as a wave where the vibrations or oscillations of the particles are parallel to the direction of wave travel (energy transfer).
(ii) Use the wave equation: \(v = f \lambda\)
Convert frequency to Hz: \(f = 40\text{ kHz} = 40\,000\text{ Hz}\)
Rearrange to solve for wavelength (\(\lambda\)): \(\lambda = \frac{v}{f} = \frac{1500}{40\,000} = 0.0375\text{ m}\)
(b)(i) Radio waves travel at the speed of light in air/vacuum: \(3.0 \times 10^8\text{ m/s}\).
(ii) Radio waves are electromagnetic waves, consisting of oscillating electric and magnetic fields, which can propagate through a vacuum without any physical medium. Ultrasound waves are sound waves, which are mechanical longitudinal waves that require a physical medium (particles) to compress and rarefy for transmission.
評分準則
(a)(i) - 1 mark: direction of vibration/oscillation of particles. - 1 mark: is parallel to the direction of energy transfer.
(ii) - 1 mark: correct formula used, \(v = f \lambda\) or \(\lambda = \frac{v}{f}\). - 1 mark: correct substitution and conversion, \(\frac{1500}{40\,000}\). - 1 mark: correct value (\(0.0375\)) and unit (\(\text{m}\) or metres).
(ii) - 1 mark: states radio waves are electromagnetic waves (and do not need a medium). - 1.88 marks: states sound/ultrasound is a mechanical wave (and requires a medium to propagate/vibrating particles).
題目 5 · structured
8.88 分
(a) A student investigates the reaction between dilute hydrochloric acid and calcium carbonate chips.
(i) Identify the gas produced and describe a chemical test, including the positive observation, to confirm its identity. [3]
(ii) Explain why the total mass of the reaction flask and its contents decreases as the reaction proceeds. [1.88]
(b) The student repeats the experiment at a higher temperature.
Explain, in terms of collision theory, why increasing the temperature increases the rate of this reaction. [4]
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解題
(a)(i) Carbon dioxide gas (\(\text{CO}_2\)) is produced. The chemical test involves bubbling the gas through limewater (aqueous calcium hydroxide). A positive result is indicated by the limewater turning cloudy or milky.
(ii) Carbon dioxide is a gas that escapes from the open reaction flask into the atmosphere. Because mass is lost to the environment as gas, the total mass of the flask and contents decreases.
(b) When the temperature increases, the reacting particles gain kinetic energy and move faster. This leads to: 1. A higher frequency of collisions (more collisions per unit time). 2. A higher proportion of colliding particles having energy equal to or greater than the activation energy (more collisions are successful).
(ii) - 1 mark: Carbon dioxide is a gas. - 0.88 marks: escapes from the flask (into the surroundings).
(b) - 1 mark: Reactant particles gain kinetic energy / move faster. - 1 mark: Greater frequency of collisions (more collisions per unit time). - 1 mark: More particles have energy greater than or equal to the activation energy. - 1 mark: Higher rate of successful/fruitful collisions.
題目 6 · structured
8.88 分
(a) Photosynthesis is the pathway by which plants synthesize carbohydrates.
(i) Write the balanced chemical equation for photosynthesis. [3]
(ii) State the name of the tissue in a leaf that is the primary site of photosynthesis, and describe one structural adaptation of its cells. [1.88]
(b) An experiment investigates the rate of photosynthesis in an aquatic plant.
Explain why increasing the temperature from \(20^\circ\text{C}\) to \(60^\circ\text{C}\) causes the rate of photosynthesis to decrease to zero, even under high light intensity. [4]
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解題
(a)(i) The balanced chemical equation for photosynthesis is: \(6\text{CO}_2 + 6\text{H}_2\text{O} \rightarrow \text{C}_6\text{H}_{12}\text{O}_6 + 6\text{O}_2\) (Light and chlorophyll may be written over the arrow, but are not reactants/products).
(ii) The palisade mesophyll is the primary tissue. Adaptations of its cells include: being vertically elongated and closely packed to absorb maximum light, and containing a very high concentration of chloroplasts.
(b) Photosynthesis is an enzyme-controlled chemical process. At \(20^\circ\text{C}\), enzymes function normally. As temperature is raised to \(60^\circ\text{C}\) (which is far above the optimum temperature), the high kinetic energy breaks the intermolecular bonds maintaining the enzymes' specific three-dimensional structures. This denatures the enzymes, changing the shape of their active sites so that the substrate molecules can no longer fit, halting the reaction entirely.
評分準則
(a)(i) - 1 mark: Correct reactants and products (\(\text{CO}_2\) and \(\text{H}_2\text{O}\) on left, \(\text{C}_6\text{H}_{12}\text{O}_6\) and \(\text{O}_2\) on right). - 1 mark: Correct balancing. - 1 mark: Condition of light and/or chlorophyll shown (either on arrow or stated in accompanying text, or full marks given if equation is perfectly balanced).
(ii) - 1 mark: Palisade mesophyll (accept palisade layer). - 0.88 marks: Contains many chloroplasts / vertically elongated / closely packed near top of leaf to maximize light absorption.
(b) - 1 mark: Photosynthesis is controlled by enzymes. - 1 mark: At high temperature (\(60^\circ\text{C}\)) enzymes denature. - 1 mark: The active site changes shape. - 1 mark: Substrates can no longer bind / fit (no enzyme-substrate complexes can form).
題目 7 · structured
8.88 分
A student investigates the propagation of water waves in a ripple tank. (a) The waves travel from a region of deep water to a region of shallow water. (i) Describe how the wavelength and the speed of the water waves change as they enter the shallow water. [2] (ii) State what happens, if anything, to the frequency of the waves. [1] (b) The speed of the water waves in the deep water is \(0.24\text{ m/s}\). The frequency of the wave source is \(6.0\text{ Hz}\). Calculate the wavelength of these waves in the deep water. Show your working and state the unit. [3] (c) Some electromagnetic waves are used in communications. (i) State which electromagnetic radiation has the lowest frequency. [1] (ii) Explain, in terms of safety, why optical fibres rather than microwaves are preferred for high-speed home internet connections, focusing on potential hazards of high-intensity exposure to microwaves. [1.88]
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解題
(a)(i) Wavelength decreases and speed decreases. (a)(ii) The frequency remains constant / does not change. (b) Use the formula \(v = f \lambda\). Rearranging gives \(\lambda = v / f = 0.24 / 6.0 = 0.040\). The unit is meters (m). (c)(i) Radio waves. (c)(ii) High-intensity microwaves can cause internal heating of body cells/tissues. Optical fibres carry light/infrared signals safely enclosed within the cable, posing no such environmental radiation hazard to humans.
評分準則
(a)(i) Wavelength decreases [1 mark] and speed decreases [1 mark]. (a)(ii) Frequency remains constant / is unchanged [1 mark]. (b) Formula stated or implied: \(v = f \lambda\) [1 mark]; Calculation: \(\lambda = 0.24 / 6.0 = 0.040\) [1 mark]; Correct unit: m or meters [1 mark] (accept \(4.0\text{ cm}\) if calculated correctly). (c)(i) Radio waves [1 mark]. (c)(ii) Statement that microwaves can cause internal heating of body tissue/cells [1 mark]; contrasting with optical fibres being safely enclosed/no radiation hazard [0.88 marks].
題目 8 · structured
8.88 分
A student investigates the rate of reaction between excess calcium carbonate chips and dilute hydrochloric acid: \(\text{CaCO}_3(\text{s}) + 2\text{HCl}(\text{aq}) \rightarrow \text{CaCl}_2(\text{aq}) + \text{H}_2\text{O}(\text{l}) + \text{CO}_2(\text{g})\). (a) The student measures the volume of carbon dioxide gas produced over time. (i) Explain why the rate of reaction decreases as the reaction progresses. Use collision theory in your answer. [3] (ii) Explain how the graph of volume of gas against time would change if the student repeated the experiment using the same mass of calcium carbonate but as a fine powder instead of chips. Explain this change using collision theory. [3] (b) Explain, using collision theory, how increasing the temperature increases the rate of this chemical reaction. [2.88]
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解題
(a)(i) As the reaction proceeds, the hydrochloric acid is used up, so the concentration of hydrogen ions decreases. This means there are fewer reactant particles per unit volume, which reduces the frequency of collisions between reactant particles. (a)(ii) Using a powder increases the surface area of calcium carbonate. This increases the frequency of collisions between acid particles and the solid reactant. The graph would have a steeper initial gradient (showing a faster rate), but would level off at the same final volume of carbon dioxide because the same mass of reactants is used. (b) Increasing the temperature increases the kinetic energy of the particles, so they move faster and collide more frequently. Crucially, a much larger proportion of colliding particles have energy equal to or greater than the activation energy, leading to a higher frequency of successful collisions.
評分準則
(a)(i) Acid particles are used up / concentration of acid decreases [1 mark]; fewer reactant particles per unit volume [1 mark]; lower frequency of collisions [1 mark]. (a)(ii) Fine powder has a larger surface area [1 mark]; leads to increased collision frequency [1 mark]; graph has a steeper initial gradient but same final volume [1 mark]. (b) Particles have more kinetic energy / move faster [1 mark]; more collisions have energy equal to or greater than the activation energy [1 mark]; higher rate of successful collisions [0.88 marks].
題目 9 · structured
8.88 分
Amylase is an enzyme that catalyses the breakdown of starch into maltose. (a) Define the term enzyme. [2] (b) An experiment was carried out to investigate the effect of temperature on the rate of starch breakdown by amylase. The rate of reaction increases from \(10^\circ\text{C}\) up to \(40^\circ\text{C}\), but then decreases rapidly and stops completely at \(65^\circ\text{C}\). (i) Explain why the rate of reaction increases as the temperature increases from \(10^\circ\text{C}\) to \(40^\circ\text{C}\). [2] (ii) Explain why the reaction stops completely at \(65^\circ\text{C}\). Use the terms active site and denatured in your answer. [3] (c) State how the activity of amylase would change if it were placed in a highly acidic solution of \(\text{pH } 2\), and explain why. [1.88]
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解題
(a) An enzyme is a protein that functions as a biological catalyst, speeding up chemical reactions without being changed or used up in the process. (b)(i) As temperature increases, the enzyme and substrate molecules gain kinetic energy, meaning they move faster. This results in a higher frequency of successful collisions between the substrate and the active site of the enzyme. (b)(ii) At \(65^\circ\text{C}\), the high temperature causes the bonds holding the enzyme's three-dimensional structure together to break. The enzyme becomes denatured, and the shape of its active site is permanently altered. The substrate can no longer fit into the active site as they are no longer complementary. (c) Amylase activity would stop or decrease significantly because a highly acidic \(\text{pH } 2\) environment denatures amylase (which normally functions near neutral pH), altering its active site so starch can no longer bind.
評分準則
(a) Protein [1 mark]; biological catalyst / speeds up reactions [1 mark]. (b)(i) Molecules gain kinetic energy / move faster [1 mark]; increased frequency of successful collisions between substrate and active site [1 mark]. (b)(ii) Enzyme is denatured [1 mark]; active site changes shape [1 mark]; substrate is no longer complementary / cannot fit into the active site [1 mark]. (c) Activity stops or decreases significantly [1 mark]; because the extreme pH denatures the enzyme / changes shape of active site [0.88 marks].
Paper 51 (Practical Test)
Practical tasks assessing experimental and planning skills. Answer all questions.
4 題目 · 40 分
題目 1 · practical
10 分
A student investigates the rate of reaction between a yeast suspension (containing the enzyme catalase) and hydrogen peroxide. The student sets up a boiling tube containing yeast and connects it to a gas syringe.
(a) (i) At 60 seconds, the plunger of the gas syringe is at 14.5 cm3. At 120 seconds, the plunger is at 28.5 cm3. State the volume of gas collected at 60 s and at 120 s. (ii) Calculate the average rate of gas production between 60 s and 120 s. Show your working and state the unit.
(b) (i) State one variable, other than concentration, that must be kept constant to ensure a fair comparison when repeating this with different concentrations of hydrogen peroxide. (ii) Explain how the variable identified in (b)(i) can be controlled.
(c) Identify the gas produced in this reaction and state the test used to confirm its identity.
(d) Suggest one potential source of experimental error in this setup and describe a modification to reduce this error.
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解題
(a) (i) Reading from the described plunger positions directly: Volume at 60 s = 14.5 cm3 Volume at 120 s = 28.5 cm3
(ii) Change in volume = \( 28.5 - 14.5 = 14.0\text{ cm}^3 \). Time interval = \( 120 - 60 = 60\text{ s} \). Average rate = \( \frac{14.0\text{ cm}^3}{60\text{ s}} = 0.23\text{ cm}^3/\text{s} \).
(b) (i) Variables to keep constant include: temperature, pH, yeast suspension concentration, or total volume of the reaction mixture. (ii) Temperature can be kept constant by placing the reaction tube in a thermostatically controlled water bath. pH can be controlled using a buffer solution.
(c) Catalase decomposes hydrogen peroxide into water and oxygen gas. The test for oxygen is inserting a glowing splint, which will relight.
(d) A common error is the escape of gas when the yeast and hydrogen peroxide are mixed before the stopper is inserted. A suitable modification is to use a flask with a side-arm containing a dropping funnel, allowing the hydrogen peroxide to be added to the yeast while the system remains completely sealed.
評分準則
(a) (i) • 14.5 cm3 [1] • 28.5 cm3 [1]
(ii) • Correct subtraction: 14.0 cm3 [1] • Division by 60 to give 0.23 (allow 0.233) with correct unit: cm3/s or cm3 s-1 [1]
(b) (i) • Temperature / volume of yeast suspension / concentration of yeast suspension / pH [1]
(ii) • Use of water bath (for temperature) / buffer (for pH) [1]
(c) • Oxygen [1] • Relights a glowing splint [1]
(d) • Error: Gas loss before stopper is replaced / variation in mixing speed [1] • Modification: Use of a dropping funnel / dividing flask / side-arm tube to mix reactants without opening the system [1]
題目 2 · practical
10 分
A student is provided with a green solid Y. They perform chemical tests to identify the ions present.
(a) (i) The student adds dilute nitric acid to a sample of solid Y in a test-tube and bubbles the gas produced through limewater. State the expected observation in the limewater. (ii) Identify the anion present in Y.
(b) The mixture from (a) is filtered to obtain a blue-green filtrate Z. The student divides filtrate Z into two separate test-tubes. (i) Describe the observations when aqueous sodium hydroxide is added dropwise and then in excess to the first test-tube. (ii) Describe the observations when aqueous ammonia is added dropwise and then in excess to the second test-tube. (iii) Identify the cation present in Z.
(c) State the chemical formula of solid Y.
(d) Describe how the student can obtain pure, dry crystals of the metal salt dissolved in filtrate Z.
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解題
(a) (i) Carbonates react with acids to release carbon dioxide gas. When carbon dioxide gas is bubbled through limewater, it turns cloudy (or milky) due to the formation of calcium carbonate precipitate.
(ii) The anion is carbonate, \( \text{CO}_3^{2-} \).
(b) (i) Copper(II) ions react with aqueous sodium hydroxide to form a light blue precipitate of copper(II) hydroxide, which is insoluble in excess sodium hydroxide.
(ii) Copper(II) ions react with aqueous ammonia to form a light blue precipitate of copper(II) hydroxide. Upon adding excess ammonia, the precipitate dissolves to form a characteristic deep blue solution containing the complex ion \( [\text{Cu}(\text{NH}_3)_4(\text{H}_2\text{O})_2]^{2+} \).
(iii) The cation is copper(II), \( \text{Cu}^{2+} \).
(c) Combining the cation \( \text{Cu}^{2+} \) and the anion \( \text{CO}_3^{2-} \) yields the chemical formula \( \text{CuCO}_3 \) (copper(II) carbonate).
(d) To obtain pure, dry crystals: heat the filtrate in an evaporating basin until the crystallization point is reached (indicated by crystals forming on a cold glass rod). Allow the solution to cool slowly so crystals grow. Filter the crystals from the remaining liquid, rinse with cold distilled water, and dry them between sheets of filter paper.
(d) • Heat filtrate to crystallization point / evaporate some water and leave to cool [1] • Filter off crystals and dry using filter paper (do not accept heating to dryness) [1]
題目 3 · practical
10 分
A student investigates the extension of a spring to determine its spring constant and the mass of an unknown stone.
(a) (i) The initial length of the unstretched spring, \( L_0 \), is 12.4 cm. When a 1.0 N load is hung, the new length, \( L_1 \), is 16.8 cm. State the values of \( L_0 \) and \( L_1 \). (ii) Calculate the extension, \( e \), caused by this 1.0 N load.
(b) (i) State the relationship between load and extension for this spring before it reaches its limit of proportionality. (ii) Use the extension from (a)(ii) to calculate the spring constant, \( k \), of the spring in N/m. Show your working.
(c) Describe two experimental precautions the student must take to ensure the length measurements of the spring are accurate.
(d) Explain how the student can use this calibrated spring to determine the mass of an unknown stone.
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解題
(a) (i) The lengths are read directly from the scale as: \( L_0 = 12.4\text{ cm} \) \( L_1 = 16.8\text{ cm} \)
(ii) Extension \( e = L_1 - L_0 = 16.8\text{ cm} - 12.4\text{ cm} = 4.4\text{ cm} \).
(b) (i) The extension is directly proportional to the applied load (Hooke's Law).
(ii) To calculate \( k \) in N/m: Extension \( e = 4.4\text{ cm} = 0.044\text{ m} \). \( k = \frac{F}{e} = \frac{1.0\text{ N}}{0.044\text{ m}} = 22.7\text{ N/m} \) (or 23 N/m).
(c) Accuracy precautions include: 1. Position the eye perpendicular to the scale of the ruler when reading (to avoid parallax error). 2. Use a set square to ensure the ruler is completely vertical and parallel to the spring.
(d) To find the mass of the stone: 1. Suspend the stone from the spring and record the new extension, \( e_{\text{stone}} \). 2. Calculate the force (weight, \( W \)) of the stone using \( W = k \times e_{\text{stone}} \) (or read the weight from a calibration curve of load against extension). 3. Use the formula \( m = \frac{W}{g} \) (where \( g = 10\text{ N/kg} \) or \( 9.8\text{ N/kg} \)) to calculate the mass in kg.
(b) (i) • Extension is directly proportional to load / weight [1]
(ii) • Conversion of 4.4 cm to 0.044 m [1] • \( k = 22.7\text{ N/m} \) (accept range 22.5 to 23.0) [1]
(c) • Read scale at eye level / perpendicular to scale to avoid parallax [1] • Ensure ruler is vertical / use a set square / use a fiducial marker [1]
(d) • Hang stone and measure its extension [1] • Calculate weight using weight = \( k \times \text{extension} \) (or use a graph) [1] • Calculate mass using \( m = \frac{W}{g} \) [1]
題目 4 · practical
10 分
A student investigates the effect of temperature on the rate of breakdown of starch by the enzyme amylase. The student uses iodine solution on a spotting tile to test for the presence of starch at regular intervals.
(a) (i) The thermometer scale has major divisions every 10 \(^{\circ}\text{C}\) and minor divisions every 1 \(^{\circ}\text{C}\). In the water bath for the first trial, the meniscus of the liquid column is exactly on the third graduation line above 20 \(^{\circ}\text{C}\). Record this temperature, \(T\).
(ii) The digital stopwatch displays "01:35" (minutes:seconds) for the reaction at 40 \(^{\circ}\text{C}\). Record this time in seconds, \(t\).
(b) The student obtains the following results for the other trials: - At 20 \(^{\circ}\text{C}\), the time taken was 4 minutes and 10 seconds. - At 30 \(^{\circ}\text{C}\), the time taken was 2 minutes and 30 seconds. - At 50 \(^{\circ}\text{C}\), the time taken was 1 minute and 45 seconds. - At 60 \(^{\circ}\text{C}\), the starch was still present after 10 minutes.
Calculate and state the times in seconds for the trials at 20 \(^{\circ}\text{C}\), 30 \(^{\circ}\text{C}\), and 50 \(^{\circ}\text{C}\).
(c) State one variable, other than temperature, that must be kept constant in this investigation, and describe how this control is achieved.
(d) Suggest why testing for the presence of starch at 30-second intervals introduces an uncertainty (error) in the determination of the exact time for starch digestion, and suggest how this error can be reduced.
(e) Describe a control experiment that the student could perform to prove that the breakdown of starch is catalysed by the active enzyme amylase and not by some other factor.
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解題
(a) (i) The meniscus is on the third graduation line above 20, so the temperature is \(23.0\text{ }^{\circ}\text{C}\) (or \(23\text{ }^{\circ}\text{C}\)). (ii) 1 minute and 35 seconds is equal to \(60 + 35 = 95\text{ s}\).
(c) Variable: Volume or concentration of starch solution (or amylase solution), or pH. How to control: Measure the volume accurately using a graduated syringe or pipette (or use a buffer solution to control pH).
(d) Uncertainty/Error: The exact end-point (when all starch is digested) could occur at any time between two consecutive 30-second tests (e.g., between 60 and 90 seconds), meaning the recorded time could be up to 30 seconds longer than the actual time. Improvement: Test for the presence of starch at shorter, more frequent intervals (e.g., every 10 seconds).
(e) Control experiment: Repeat the experiment using boiled (denatured) amylase solution (or distilled water) instead of active amylase solution, keeping all other conditions (volume, temperature, pH) identical.
評分準則
(a) (i) [1 mark] for \(23\text{ }^{\circ}\text{C}\) or \(23.0\text{ }^{\circ}\text{C}\). (a) (ii) [1 mark] for \(95\text{ s}\). (b) [1 mark] for correct conversion of 20 \(^{\circ}\text{C}\) trial (\(250\text{ s}\)) AND [1 mark] for correct conversion of 30 \(^{\circ}\text{C}\) (\(150\text{ s}\)) and 50 \(^{\circ}\text{C}\) (\(105\text{ s}\)) trials. (c) [1 mark] for identifying a correct constant variable (e.g., volume/concentration of starch/amylase, or pH) AND [1 mark] for matching method of control (e.g., using a syringe/pipette to measure volume, or using a buffer solution for pH). (d) [1 mark] for explaining that the actual endpoint lies between the 30-second sampling intervals AND [1 mark] for suggesting testing at shorter/more frequent intervals (e.g., every 10 seconds). (e) [1 mark] for using boiled/denatured amylase (or water/buffer instead of enzyme) AND [1 mark] for keeping all other factors (temperature, volumes, concentration) constant.
Paper 61 (Alternative to Practical)
Written questions assessing experimental design, analysis, and planning skills. Answer all questions.
4 題目 · 40 分
題目 1 · practical
10 分
A student investigates the effect of temperature on the rate of the breakdown of starch by amylase.
At five different temperatures, the student mixes amylase and starch solutions. Every 30 seconds, a sample of the mixture is added to a drop of iodine solution on a spotting tile. The student records the time taken for the iodine solution to stop turning blue-black (i.e. to remain yellow-brown).
(a) (i) Fig. 1.1 shows a thermometer placed in a water bath. The liquid level is exactly halfway between the 38 °C and 39 °C markings. Record this temperature to the nearest 0.5 °C. (ii) Fig. 1.2 shows the stopclock reading when the iodine solution remains yellow-brown at this temperature. The stopclock shows 02:15 (2 minutes and 15 seconds). Calculate this time in seconds.
(b) State two variables, other than the volume of amylase solution, that must be kept constant in this investigation to ensure a fair test.
(c) State why the starch solution and the amylase solution are left in the water bath for 5 minutes before they are mixed together.
(d) Explain why the iodine solution remains yellow-brown at the end of the reaction instead of turning blue-black.
(e) Describe a control experiment that the student could perform to prove that it is the active enzyme amylase that is responsible for the starch breakdown.
(f) State one hazard associated with using a hot water bath and describe a safety precaution to minimise this risk.
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解題
(a) (i) The thermometer level is exactly halfway between 38 °C and 39 °C, so the temperature is 38.5 °C. (ii) 2 minutes is 120 seconds. 120 s + 15 s = 135 s. (b) Controlling other factors (such as starch concentration/volume and pH) ensures that any change in reaction rate is solely due to the change in temperature. (c) Pre-incubating the reactants ensures they are at the correct target temperature when the reaction begins. (d) Iodine only turns blue-black in the presence of starch. If the amylase has fully broken down the starch, no starch remains, so the iodine stays yellow-brown. (e) A control experiment replaces the active enzyme with an inactive substance (boiled amylase) to show that without active enzyme, the starch does not break down. (f) Hot water is a burn hazard. Using test-tube holders or heat-resistant gloves protects the skin.
評分準則
Total Marks: 10
(a) (i) 38.5 (°C) [1] (Accept 38.5 without unit, reject 38 or 39) (a) (ii) 135 (s) [1] (b) Any two from: volume of starch, concentration of starch, concentration of amylase, pH [2] (1 mark each) (c) To allow the solutions to reach the temperature of the water bath [1] (d) Starch has been completely broken down / digested / hydrolysed [1] (e) Use boiled/denatured amylase OR use water instead of amylase [1]; observe that the starch is not broken down / iodine continues to turn blue-black [1] (f) Hazard: Hot water / hot glassware can cause burns [1]; Precaution: Wear insulated gloves / use tongs / use a water bath holder [1] (Precaution must match the stated hazard)
題目 2 · practical
10 分
A student investigates the rate of reaction between calcium carbonate (marble chips) and dilute hydrochloric acid. The reaction produces carbon dioxide gas.
The student collects the gas in a gas syringe and records the volume of gas collected every 20 seconds.
(a) Fig. 2.1 shows the gas syringe reading at 60 seconds. The plunger of the syringe points exactly to the fourth small division between 30 cm³ and 40 cm³. Each small division represents 1 cm³. Record the volume of gas collected at 60 seconds.
(b) (i) Explain, in terms of concentration of reactants, why the rate of reaction is fastest at the start of the reaction and decreases over time. (ii) State how the student would know from the readings when the reaction has completely stopped.
(c) Describe a chemical test to confirm that the gas collected in the syringe is carbon dioxide, including the positive result and the name of the precipitate formed.
(d) The student wants to repeat this investigation at a higher temperature. (i) State the effect of increasing temperature on the rate of reaction. (ii) Explain this effect in terms of collision theory.
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解題
(a) The syringe scale shows divisions of 1 cm³. Four divisions above 30 cm³ is 34 cm³. (b) (i) Initially, the concentration of hydrochloric acid is at its maximum, meaning there are more reactant particles per unit volume, which maximizes the frequency of collisions. As reactants are converted to products, the concentration of acid decreases, reducing collision frequency and thus the rate. (ii) When the reaction stops, no more carbon dioxide is produced, so the volume reading on the syringe remains constant. (c) Carbon dioxide reacts with calcium hydroxide (limewater) to form an insoluble precipitate of calcium carbonate, turning the solution cloudy. (d) (i) Raising the temperature increases the rate of reaction. (ii) At higher temperatures, particles have more kinetic energy. This leads to more frequent collisions, and a greater fraction of colliding particles have energy exceeding the activation energy, increasing the rate of successful collisions.
評分準則
Total Marks: 10
(a) 34 (cm³) [1] (Accept 34.0) (b) (i) Highest concentration of reactant particles at start / more frequent collisions [1]; reactants are used up / concentration decreases, leading to less frequent collisions [1] (b) (ii) The volume of gas stops increasing / remains constant over time [1] (c) Limewater [1]; turns cloudy / milky / chalky [1]; precipitate is calcium carbonate [1] (d) (i) Rate of reaction increases [1] (d) (ii) Particles have more kinetic energy / move faster [1]; leading to more frequent collisions OR more particles have energy greater than/equal to activation energy / more successful collisions [1]
題目 3 · practical
10 分
A student investigates the path of a ray of light passing through a rectangular glass block to determine its refractive index.
The student draws the outline of the glass block on a piece of paper. A light ray is directed at the block, and the student marks the path of the incident and emergent rays.
(a) The student measures the angle of incidence \(i\) to be 48° and the angle of refraction \(r\) inside the block to be 30°. (i) State the values of \(\sin 48^\circ\) and \(\sin 30^\circ\). (ii) Calculate the refractive index \(n\) of the glass block using the formula:
\[n = \frac{\sin i}{\sin r}\]
Show your working.
(b) State two reasons why using thin pencil lines and a narrow ray of light improves the accuracy of this practical.
(c) Describe how the student can trace the exact path of the light ray inside the glass block without drawing lines on the glass itself.
(d) The student wants to investigate if the refractive index of the glass block is different for different colours of light. (i) Describe how the student would modify the investigation to test this. (ii) State two key variables that must be kept constant to ensure a fair comparison.
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解題
(a) (i) \(\sin 48^\circ \approx 0.743\) (rounded to 0.74) and \(\sin 30^\circ = 0.50\). (ii) Substituting these values into the formula gives \(n = \frac{0.74}{0.50} = 1.48\). If using more precise values, \(\frac{0.743}{0.5} = 1.49\). (b) A thick light beam makes it difficult to locate the exact center of the ray, leading to errors when drawing lines and measuring angles. Thin pencil lines minimize this uncertainty. (c) Light travels in straight lines inside the glass. By marking where the ray enters the block and where it exits, the path inside can be reconstructed by joining these marks with a straight line after the block is removed. (d) (i) Color of light is determined by its wavelength. By using different filters (red, green, blue) on the white light source, the student can measure the refractive index for each color. (ii) To isolate color as the independent variable, the student must keep the block material (the same block) and the angle of incidence constant.
評分準則
Total Marks: 10
(a) (i) \(\sin 48^\circ = 0.74\) (accept 0.743) AND \(\sin 30^\circ = 0.5\) (accept 0.50) [1] (a) (ii) Correct substitution of values: \(1.48\) / \(1.49\) / \(1.5\) [1]; correct working shown [1] (b) Easier to find the center of the light ray / less uncertainty in drawing lines [1]; allows more accurate reading of angles with a protractor [1] (c) Mark the entry and exit points on the block's outline [1]; remove the block and connect these two points with a straight line using a ruler [1] (d) (i) Use different colored filters / light sources / lasers [1] (d) (ii) Use the same glass block / same block material [1]; keep the angle of incidence constant [1]
題目 4 · practical
10 分
A student investigates the rate of reaction between dilute hydrochloric acid and zinc pieces. They use a conical flask connected to a gas syringe to collect and measure the volume of hydrogen gas produced.
In each experiment, the student uses the same mass of zinc and a fixed volume of acid at room temperature. They record the volume of gas collected after exactly 2 minutes (120 seconds).
(a) The scale on the gas syringe is marked every \(10\text{ cm}^3\), with small divisions every \(2\text{ cm}^3\). (i) For \(0.5\text{ mol/dm}^3\) acid, the volume of gas collected is \(18\text{ cm}^3\). - For \(1.0\text{ mol/dm}^3\) acid, the syringe plunger rests on the third small division mark above the \(30\text{ cm}^3\) line. - For \(1.5\text{ mol/dm}^3\) acid, the syringe plunger rests on the second small division mark above the \(50\text{ cm}^3\) line.
State the volume of gas collected for the \(1.0\text{ mol/dm}^3\) and \(1.5\text{ mol/dm}^3\) concentrations.
(ii) Calculate the average rate of reaction in \(\text{cm}^3/\text{s}\) during the 2-minute period for the \(1.5\text{ mol/dm}^3\) acid.
(b) Identify two variables, other than the concentration of acid and the time, that the student must keep constant to ensure a valid comparison (fair test).
(c) Explain, using particle collision theory, why increasing the concentration of hydrochloric acid increases the rate of reaction.
(d) During the setup, some hydrogen gas may escape before the stopper (bung) is inserted into the flask. Suggest an experimental improvement to prevent this loss of gas at the start of the reaction.
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解題
(a)(i) - For the \(1.0\text{ mol/dm}^3\) acid, the plunger is at 3 divisions above 30. Each division is \(2\text{ cm}^3\), so: \(30 + (3 \times 2) = 36\text{ cm}^3\). - For the \(1.5\text{ mol/dm}^3\) acid, the plunger is at 2 divisions above 50: \(50 + (2 \times 2) = 54\text{ cm}^3\).
(a)(ii) - Time = 2 minutes = 120 seconds. - Average rate of reaction = \(\frac{\text{Volume of gas}}{\text{Time}} = \frac{54\text{ cm}^3}{120\text{ s}} = 0.45\text{ cm}^3/\text{s}\).
(b) To ensure a fair test, all other factors affecting rate must be kept constant: - The mass of zinc used (determines the potential amount of reactant). - The surface area / particle size of the zinc (e.g., using granules of the same size, not powder vs. large lumps). - The temperature of the reaction mixture (temperature changes kinetic energy of particles). - The volume of acid used (to keep stoichiometry comparable).
(c) According to collision theory, higher concentration means more reactant particles in a given volume. This results in more frequent collisions (more collisions per second) between the zinc and the acid particles, increasing the rate of reaction.
(d) When zinc is added by hand, gas escapes in the brief moment before the bung is pushed in. An improvement is to keep the reactants separate inside a closed system. For example, suspending the zinc on a thread inside the flask and dropping it by releasing the thread while the bung is already sealed, or using a side-arm test tube / divided flask where tilting mixes the reactants.
評分準則
Part (a)(i) [2 marks total]: - \(36\text{ cm}^3\) [1 mark] - \(54\text{ cm}^3\) [1 mark]
Part (a)(ii) [2 marks total]: - Conversion of 2 minutes to 120 seconds [1 mark] - Correct calculation of rate: \(0.45\text{ cm}^3/\text{s}\) [1 mark]
Part (b) [2 marks total]: - Any two correct control variables: mass of zinc, surface area of zinc, temperature of acid, volume of acid [1 mark each]. - Reject: 'amount of zinc' unless specified as mass or surface area.
Part (c) [2 marks total]: - States there are more particles per unit volume / space [1 mark] - States this increases the collision frequency / rate of collisions / collisions per unit time [1 mark] (Reject: 'more collisions' without a time reference like 'frequency' or 'per second').
Part (d) [2 marks total]: - Identifies a mechanism to mix reactants without opening the system (e.g., suspending zinc on a thread, using a side-arm flask / reaction cup) [1 mark] - Explains how this prevents gas escaping (e.g., flask is fully sealed before reaction begins) [1 mark].
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