2025 Cambridge IGCSE Science - Combined (0653) 模擬試題連答案詳解

A student listens to a sound wave produced by a loudspeaker. The volume of the sound is increased, but the pitch of the sound remains unchanged. How does the amplitude and the frequency of the sound wave change?

The table shows some properties of the Group I elements lithium and sodium. Potassium is directly below sodium in Group I. Element | Melting point in degrees Celsius | Reactivity with water. Lithium: 181, reacts steadily. Sodium: 98, reacts vigorously. Potassium: x, y. Which row in the table below shows the correct predicted values of x and y?

What are the main substances transported by xylem and phloem vessels in a plant?

Which statement correctly compares the approximate percentage concentrations of oxygen and carbon dioxide in inspired air and expired air?

A neutral atom of sodium has a proton number of 11 and a nucleon number of 23. What is the correct number of protons, neutrons and electrons in this atom?

A circuit consists of a battery connected in series with an ammeter and two identical lamps. If the ammeter reads 0.4 A, what is the current flowing through each of the two lamps?

Which feature of gas exchange surfaces in humans decreases the distance over which gases must diffuse?

A cyclist travels a distance of \(1200\text{ m}\) in a total time of \(3.0\text{ minutes}\). What is the average speed of the cyclist?

The chemical equation represents a reaction between copper(II) oxide and hydrogen: \(\text{CuO} + \text{H}_2 \rightarrow \text{Cu} + \text{H}_2\text{O}\). Which statement about this reaction is correct?

The table compares the approximate percentage concentration of oxygen and carbon dioxide in inspired air and expired air.

Which row is correct?

| | Percentage of oxygen in inspired air | Percentage of oxygen in expired air | Percentage of carbon dioxide in inspired air | Percentage of carbon dioxide in expired air |
|---| :---: | :---: | :---: | :---: |
| **A** | 21% | 16% | 0.04% | 4% |
| **B** | 16% | 21% | 0.04% | 4% |
| **C** | 21% | 16% | 4% | 0.04% |
| **D** | 16% | 21% | 4% | 0.04% |

A student reacts calcium carbonate chips with dilute hydrochloric acid. Which change would decrease the initial rate of production of carbon dioxide gas?

In a series circuit, a cell is connected to two identical resistors. The current leaving the cell is \(0.6\text{ A}\). The electromotive force (e.m.f.) of the cell is \(3.0\text{ V}\).

What is the current in each resistor and the potential difference (p.d.) across each resistor?

A plant cell is placed in a concentrated sucrose solution. Which row correctly describes the net movement of water and the effect on the cell membrane?

In the extraction of iron from hematite in a blast furnace, which raw material is added to react with and remove acidic impurities?

A student of mass \(50\text{ kg}\) climbs a flight of stairs. The vertical height of the stairs is \(4.0\text{ m}\) and the student takes \(8.0\text{ s}\) to climb them. The acceleration of free fall, \(g\), is \(10\text{ m/s}^2\). What is the average power developed by the student?

A student exhales through a tube into a flask containing clear limewater. After a few breaths, the limewater turns cloudy. What is the identity of the gas that causes this change, and how does its concentration in expired air compare to inspired air?

A student connects a 6.0 V battery in series with an ammeter and two resistors. The resistances of the resistors are 2.0 \(\Omega\) and 4.0 \(\Omega\). What is the reading on the ammeter?

Which statement about all enzymes is correct?

Four different metals, W, X, Y and Z, are added separately to test-tubes containing dilute hydrochloric acid. The observations are recorded below:

- Metal W: Bubbles of gas are produced very rapidly.
- Metal X: No reaction occurs.
- Metal Y: Bubbles of gas are produced slowly.
- Metal Z: Bubbles of gas are produced moderately fast.

What is the order of reactivity of the metals, starting with the most reactive?

Which statement correctly describes a difference between aerobic respiration and anaerobic respiration in humans?

Molten lead(II) bromide is electrolyzed using inert electrodes. What are the products formed at each electrode?

A student measures the speed of sound through three different media: air, steel, and water. Which list shows these media in order of the speed of sound through them, from slowest to fastest?

What are the products of anaerobic respiration in yeast cells?

Four metals, \(W\), \(X\), \(Y\) and \(Z\), are tested with water, steam and dilute hydrochloric acid.

- Metal \(W\) reacts slowly with cold water.
- Metal \(X\) does not react with cold water but reacts with steam.
- Metal \(Y\) reacts very rapidly with cold water.
- Metal \(Z\) does not react with steam and does not react with dilute acid.

What is the order of reactivity of the metals, from most reactive to least reactive?

A circuit contains a \(12\text{ V}\) battery, a \(4\ \Omega\) resistor, and a \(2\ \Omega\) resistor connected in series.

What is the current in the circuit and the potential difference across the \(2\ \Omega\) resistor?

A student breathes in and out through an apparatus designed to compare the carbon dioxide content of inspired and expired air. Inspired air passes through flask 1 containing limewater, and expired air passes through flask 2 containing limewater. Which observation and explanation are correct?

The elements lithium, sodium and potassium are in Group I of the Periodic Table. Which statement describes the trends in reactivity with water and melting point going down Group I from lithium to potassium?

Which row correctly identifies a longitudinal wave and describes the direction of its vibrations relative to the direction of energy transfer?

Which structure is present in both a typical plant cell and a typical animal cell, and what is its function?

Some metals are extracted from their metal oxides by heating with carbon. Which metal cannot be extracted from its oxide by heating with carbon?

Which statement about electromagnetic waves is correct?

Which statement about the trends in Group I elements (alkali metals) is correct as the group is descended from lithium to potassium?

An astronaut has a mass of \(70\text{ kg}\) on Earth, where the gravitational field strength is \(10\text{ N/kg}\). The astronaut travels to the Moon, where the gravitational field strength is \(1.6\text{ N/kg}\). What are the mass and the weight of the astronaut on the Moon?

How does the composition of expired air compare with the composition of inspired air?

Which structures are present in a plant cell but are not present in an animal cell?

Which statement about the Group I elements lithium, sodium and potassium is correct?

A resistor is connected to a power supply. The current in the resistor is 2.0 A and the potential difference across it is 6.0 V. What is the resistance of the resistor?

Four identical potato cylinders were weighed and placed into separate sucrose solutions of different concentrations: P, Q, R and S. After two hours, they were removed and reweighed. The percentage changes in mass of the potato cylinders were: Cylinder in solution P: -8%; Cylinder in solution Q: +12%; Cylinder in solution R: 0%; Cylinder in solution S: +4%. Which sucrose solution has the highest water potential?

Four identical potato cylinders were weighed and placed into separate sucrose solutions of different concentrations: P, Q, R and S. After two hours, they were removed and reweighed. The percentage changes in mass of the potato cylinders were: Cylinder in solution P: -8%; Cylinder in solution Q: +12%; Cylinder in solution R: 0%; Cylinder in solution S: +4%. Which sucrose solution has the highest water potential?

An electric motor lifts a load of mass \( 40\text{ kg} \) vertically upwards through a height of \( 12\text{ m} \) in a time of \( 6.0\text{ s} \). The gravitational field strength, \( g \), is \( 10\text{ N/kg} \). The motor has an efficiency of \( 80\% \). What is the electrical power input to the motor?

Magnesium reacts with aqueous copper(II) ions according to the equation: \( \text{Mg(s)} + \text{Cu}^{2+}\text{(aq)} \rightarrow \text{Mg}^{2+}\text{(aq)} + \text{Cu(s)} \) Which statement correctly describes what happens to the magnesium atoms during this reaction?

A student investigates the rate of photosynthesis of an aquatic plant. At a constant temperature of \( 20^\circ\text{C} \) and a high light intensity, the rate of photosynthesis is higher when the carbon dioxide concentration is \( 0.15\% \) compared to when it is \( 0.04\% \). Which factor limits the rate of photosynthesis at high light intensity when the carbon dioxide concentration is \( 0.04\% \)?

An experiment is carried out to investigate the rate of an enzyme-catalysed reaction at different temperatures. At \(50\ ^\circ\text{C}\), the rate of reaction decreases rapidly compared to the rate at \(37\ ^\circ\text{C}\).

Which statement correctly explains this observation?

During the electrolysis of molten lead(II) bromide, \(\text{PbBr}_2\), using inert graphite electrodes, what are the products formed at each electrode and the visible observations?

A student places a beaker of water on a tripod and heats it from the bottom with a Bunsen burner. A small crystal of potassium manganate(VII) is placed at the bottom of the beaker to act as a dye, showing the movement of the water.

Which statement correctly explains the formation of the convection current?

A car travels along a straight road. It accelerates uniformly from rest to a speed of \(12\text{ m/s}\) in \(4.0\text{ s}\), then continues at this constant speed of \(12\text{ m/s}\) for a further \(6.0\text{ s}\).

What is the average speed of the car for the entire \(10.0\text{ s}\) journey?

An experiment is carried out where an excess of calcium carbonate chips is reacted with a fixed volume of dilute hydrochloric acid.

Which change to the reaction conditions will increase the initial rate of reaction but **not** increase the final volume of carbon dioxide gas collected?

Which statement correctly describes the effect of temperature on enzyme-controlled reactions?

A ray of light in air is incident on a transparent glass block. The angle of incidence is \(40^\circ\) and the angle of refraction in the glass is \(25^\circ\). What is the refractive index of the glass?

An enzyme-catalyzed reaction is carried out at \(37^\circ\text{C}\). The reaction mixture is then heated to \(80^\circ\text{C}\) for 10 minutes, before being cooled back down to \(37^\circ\text{C}\). What happens to the rate of the reaction when the mixture is cooled back to \(37^\circ\text{C}\) compared to the initial rate at \(37^\circ\text{C}\)?

Four metals, W, X, Y, and Z, are tested to determine their chemical reactivity: - Metal W reacts vigorously with cold water. - Metal X does not react with cold water but reacts with steam. - Metal Y does not react with dilute hydrochloric acid. - Metal Z reacts slowly with dilute hydrochloric acid but does not react with steam. What is the correct order of reactivity of the metals, from most reactive to least reactive?

A ray of light has a frequency of \(5.0 \times 10^{14}\text{ Hz}\) in air. It enters a glass block where the speed of light is \(2.0 \times 10^8\text{ m/s}\). What is the frequency and the wavelength of this light inside the glass block? (The speed of light in air is \(3.0 \times 10^8\text{ m/s}\).)

An excess of dilute hydrochloric acid is reacted with 2.0 g of calcium carbonate chips. Which change decreases the initial rate of reaction but keeps the total volume of carbon dioxide gas produced the same?

Which row correctly describes the state of the valves in the human heart when the ventricles are contracting?

Concentrated aqueous sodium chloride is electrolysed using inert carbon electrodes. Which row correctly identifies the products formed at each electrode and the substance remaining in the solution?

The Sun is a star that radiates vast amounts of energy. Which row correctly describes the main energy-releasing process in the Sun and the primary compositional change resulting from this process?

An experiment is carried out to investigate the effect of pH on the rate of an enzyme-catalysed reaction. The enzyme has an optimum pH of 8.0. Which statement correctly explains why the rate of reaction is much lower at pH 2.0 than at pH 8.0?

A toy car of mass 0.5 kg is accelerated from rest to a final speed of 4 m/s. The car travels a distance of 2 m during this acceleration. What is the average force acting on the car in the direction of its motion?

Equal volumes of orange-brown aqueous bromine are added to two separate test-tubes in a dark room. Test-tube 1 contains hexane, and Test-tube 2 contains hexene. Both test-tubes are shaken and kept in the dark. What are the final observations in both test-tubes?

Which row correctly compares the thickness of the wall, the size of the lumen, and the relative pressure of blood in a typical artery compared with a typical vein?

An electromagnetic wave has a frequency of \( 6.0 \times 10^{14} \text{ Hz} \) in a vacuum. The speed of electromagnetic waves in a vacuum is \( 3.0 \times 10^8 \text{ m/s} \). What is the wavelength of this wave?

Which row correctly identifies the catalyst and the temperature used in the industrial manufacture of ethanol from ethene and steam?

An enzyme-catalysed reaction is investigated at different temperatures. Which statement correctly explains why the rate of reaction increases between \( 20^\circ\text{C} \) and \( 40^\circ\text{C} \), but drops to zero at \( 70^\circ\text{C} \)?

Two identical resistors, each of resistance \(R\), are connected in parallel with each other. This parallel combination is then connected in series with a third identical resistor of resistance \(R\). If the potential difference across the parallel combination is \(V_p\) and the potential difference across the third single resistor is \(V_s\), what is the ratio \(\frac{V_p}{V_s}\)?

Propene, \(\text{C}_3\text{H}_6\), is an unsaturated hydrocarbon. It reacts with aqueous bromine in an addition reaction. What is the molecular formula of the organic product formed in this reaction?

An enzyme-controlled reaction is carried out at different pH values, with all other variables kept constant. The time taken for the substrate to be completely broken down is measured and recorded: pH 4 takes 15.0 minutes, pH 6 takes 5.0 minutes, pH 8 takes 1.5 minutes, and pH 10 shows no reaction after 30 minutes. Which statement explains the result at pH 10?

A plant's root hair cells are exposed to a chemical inhibitor that completely prevents aerobic respiration. How does this treatment affect the uptake of nitrate ions and the uptake of water by the root hair cells?

Molten lead(II) bromide, \(\text{PbBr}_2\), is electrolysed using inert carbon electrodes. Which row correctly identifies the products and the observations at each electrode?

An electromagnetic wave has a frequency of \(5.0 \times 10^{14}\text{ Hz}\) in a vacuum. What is the wavelength of this wave in a vacuum?

A ray of monochromatic light travels from air into a flat glass block. Which row correctly describes how the frequency, wavelength, and speed of the light wave change as it enters the glass?

Which statement describes the trends in the physical and chemical properties of the Group VII elements (halogens) as the group is descended from chlorine to iodine?

Root hair cells absorb nitrate ions from the soil by active transport. Which of the following changes would result in a decrease in the rate of nitrate ion uptake by these cells? 1. Decreasing the temperature of the soil. 2. Adding a chemical that inhibits aerobic respiration. 3. Increasing the oxygen concentration in the soil.

An enzyme-catalysed reaction is carried out at various temperatures. It is observed that the rate of reaction increases up to an optimum temperature of \(40\ ^\circ\text{C}\), but then decreases rapidly at temperatures above \(45\ ^\circ\text{C}\).

Which statement explains why the rate of reaction decreases rapidly above the optimum temperature?

A student electrolyses concentrated aqueous sodium chloride using inert platinum electrodes.

Which row correctly identifies the products formed and the observations made at each electrode?

Water waves travel across a ripple tank from a region of deep water into a region of shallow water.

Which row correctly describes how the speed, frequency, and wavelength of the waves change as they enter the shallow water?

An enzyme-catalysed reaction is carried out at different temperatures. Which statement about the molecular events at different temperatures is correct?

Copper(II) oxide reacts with dilute sulfuric acid as shown in the equation: \(\text{CuO} + \text{H}_2\text{SO}_4 \rightarrow \text{CuSO}_4 + \text{H}_2\text{O}\). What mass of copper(II) sulfate (\(M_{\text{r}} = 160\)) is obtained when \(4.0\text{ g}\) of copper(II) oxide (\(M_{\text{r}} = 80\)) reacts completely with excess dilute sulfuric acid?

A sound wave travelling in water has a frequency of \(2.0\text{ kHz}\) and a wavelength of \(75\text{ cm}\). What is the speed of this wave in water?

An electrical appliance is connected to a \(12\text{ V}\) d.c. power supply. The current in the appliance is \(2.5\text{ A}\). How much electrical energy is transferred by the appliance in \(5.0\text{ minutes}\)?

A cyclist starts from rest and accelerates to a speed of \(6\text{ m/s}\) in \(4\text{ s}\). She then travels at this constant speed of \(6\text{ m/s}\) for another \(10\text{ s}\).

(a) (i) Describe the motion of the cyclist between \(0\text{ s}\) and \(4\text{ s}\). [1]
(ii) Calculate the distance travelled by the cyclist during the first \(4\text{ s}\). Show your working. [2]
(iii) Calculate the acceleration of the cyclist during the first \(4\text{ s}\). State the unit. [2]

(b) The combined mass of the cyclist and her bicycle is \(80\text{ kg}\).
(i) Calculate the combined weight of the cyclist and her bicycle on Earth, where the gravitational field strength \(g = 10\text{ N/kg}\). [2]
(ii) State the form of energy stored in the cyclist's muscles that is transferred to kinetic energy as she pedals. [1]
(iii) As she pedals, some energy is transferred to the surroundings as thermal energy. State the law of conservation of energy to explain why this energy is not lost. [1]

This question is about elements in the Periodic Table.

(a) Lithium, sodium and potassium are elements in Group I of the Periodic Table.
(i) Describe the trend in the reactivity of Group I elements down the group. [1]
(ii) A small piece of sodium is added to a trough of cold water. State two observations that can be made during this reaction. [2]
(iii) Complete the word equation for the reaction of sodium with water:
\(\text{sodium} + \text{water} \rightarrow \text{\_\_\_\_\_\_\_\_\_\_\_\_\_} + \text{\_\_\_\_\_\_\_\_\_\_\_\_\_}\) [2]

(b) Chlorine is an element in Group VII of the Periodic Table.
(i) State the physical state and the colour of chlorine at room temperature and pressure. [2]
(ii) Explain, in terms of electronic configuration, why helium (a noble gas in Group VIII) is very unreactive. [1]
(iii) State one use of argon. [1]

The human gas exchange system is adapted to allow efficient exchange of gases between the lungs and the blood.

(a) Air travels from the larynx down into the lungs.
(i) Name the structure that connects the larynx to the bronchi. [1]
(ii) This structure contains incomplete rings of cartilage. State the function of these cartilage rings. [1]

(b) Alveoli are the tiny air sacs where gas exchange takes place.
(i) Describe three features of alveoli that make them efficient gas exchange surfaces. [3]
(ii) Name the process by which oxygen molecules move from the alveoli into the blood capillaries. [1]

(c) A student carries out an experiment to compare inspired (inhaled) air and expired (exhaled) air.
(i) Describe how the percentage of oxygen in expired air compares to the percentage of oxygen in inspired air. [1]
(ii) Describe a chemical test to show that expired air contains more carbon dioxide than inspired air. Include the observation for a positive result. [2]

A student standing 165 m from a large, flat vertical wall claps their hands and hears an echo.

(a) Explain how the echo is produced. [2]

(b) The student hears the echo 1.0 s after clapping. Show that the speed of sound in air is approximately 330 m/s. Show your working clearly. [3.8]

(c) Sound is a longitudinal wave. Describe how a longitudinal wave differs from a transverse wave in terms of particle vibration. [3]

Green plants manufacture their own food using the process of photosynthesis.

(a) Write down the word equation for photosynthesis. [3]

(b) Chlorophyll is essential for this process. State the role of chlorophyll and identify the plant cell organelle where it is located. [2.8]

(c) Carbon dioxide is a reactant in photosynthesis. Explain how carbon dioxide from the atmosphere enters the cells inside the leaf. [3]

A student investigates the rate of reaction between dilute hydrochloric acid and calcium carbonate (marble) chips.

(a) Identify the gas produced during this reaction, and describe the chemical test used to confirm its identity. [2.8]

(b) The student repeats the experiment using hydrochloric acid of a higher concentration. State the effect of this change on the rate of reaction, and explain your answer in terms of particle collisions. [3]

(c) Next, the student uses the same mass of calcium carbonate as a fine powder instead of large chips. State and explain how this change affects the rate of reaction. [3]

A toy car moves along a straight, horizontal track.

(a) The car travels a distance of 18 meters in a time of 6.0 seconds. Calculate the average speed of the toy car. State the correct unit. [2]

(b) The mass of the toy car is 0.25 kg. Calculate the weight of the toy car. (Gravitational field strength, \(g = 10\text{ N/kg}\)). [2]

(c) State the difference between mass and weight. [2]

(d) A small plastic block used in making the car has a mass of 45 g and a volume of \(50\text{ cm}^3\). Calculate the density of the plastic block. State the unit. [3]

A student is given a mixture containing dry sand, salt, and water. They want to separate the mixture to obtain dry sand, dry salt crystals, and pure liquid water.

(a) Describe how the student can separate the sand from the salt water mixture. Name the technique and explain how it separates the sand. [3]

(b) Describe how the student can obtain pure liquid water from the remaining salt water. State the name of this process and how the apparatus collects the liquid. [3]

(c) Paper chromatography can be used to analyze food dyes.

(i) Explain why the starting line is drawn in pencil rather than ink. [1]

(ii) State where the solvent level must be placed relative to the starting line at the beginning of the experiment. [1]

The gas exchange system of a human is adapted to allow efficient transport of oxygen and carbon dioxide.

(a) Describe how the concentrations of oxygen, carbon dioxide, and nitrogen differ between inspired and expired air. Support your answers with approximate percentage values for each gas. [3]

(b) Explain how goblet cells and ciliated cells work together to protect the respiratory tract from pathogens and dust. [4]

(c) Carbon dioxide is a waste product of aerobic respiration.

(i) State the chemical used to test for the presence of carbon dioxide. [1]

(ii) State the observation for a positive result. [1]

A student investigates the rate of decomposition of hydrogen peroxide, \(\text{H}_2\text{O}_2\), to form water and oxygen gas, using manganese(IV) oxide as a catalyst.

(a) State the role of a catalyst in terms of the activation energy of a chemical reaction. [2]

(b) Describe and explain how the rate of this reaction changes over time, from the start until the reaction ceases. [3.8]

(c) The reaction is repeated at a higher temperature. Explain, in terms of collision theory, why increasing the temperature increases the rate of reaction. [3]

An electrical circuit contains a 12.0 V d.c. power supply connected to a network of resistors. A 6.0 \(\Omega\) resistor is connected in series with a parallel combination of two identical 4.0 \(\Omega\) resistors.

(a) Calculate the combined resistance of the two 4.0 \(\Omega\) resistors connected in parallel. [2]

(b) Determine the total resistance of the entire circuit. [1.8]

(c) Calculate the total current flowing from the power supply. State the unit. [2]

(d) Calculate the potential difference across the 6.0 \(\Omega\) resistor. [3]

The human gas exchange system is adapted to maximize the rate of diffusion of oxygen and carbon dioxide.

(a) State three features of the alveoli that adapt them for efficient gas exchange. [3]

(b) Explain why an athlete's breathing rate and depth of breathing increase during a race. [3.8]

(c) Name the waste gas excreted by the lungs, and state how the majority of this gas is transported from respiring tissues to the lungs. [2]

An electric heater circuit contains two identical resistors, each of resistance \(R = 12\ \Omega\), connected in parallel to a \(24\text{ V}\) power supply.

(a) State the relationship between the total current in a parallel circuit and the individual branch currents. [1 mark]

(b) Calculate the total equivalent resistance of the two parallel-connected resistors. Show your working. [2.8 marks]

(c) (i) Calculate the total electrical power dissipated by the heater when connected to the \(24\text{ V}\) supply. Show your working and state the units. [3 marks]
(ii) Describe the advantage of connecting these heating elements in parallel rather than in series in terms of reliability. [2 marks]

The rate of reaction between solid calcium carbonate and dilute hydrochloric acid can be investigated by measuring the volume of carbon dioxide gas produced over time.

\(\text{CaCO}_3(\text{s}) + 2\text{HCl}(\text{aq}) \rightarrow \text{CaCl}_2(\text{aq}) + \text{H}_2\text{O}(\text{l}) + \text{CO}_2(\text{g})\)

(a) Define the term activation energy. [1.8 marks]

(b) Explain, using collision theory, why increasing the temperature of the hydrochloric acid increases the rate of this chemical reaction. [4 marks]

(c) Calculate the mass of calcium carbonate required to produce \(120\text{ cm}^3\) of carbon dioxide gas at room temperature and pressure (r.t.p.).
[The molar volume of any gas at r.t.p. is \(24\text{ dm}^3/\text{mol}\). Relative atomic masses: \(A_r: \text{Ca} = 40, \text{C} = 12, \text{O} = 16\).] [3 marks]

Plants produce glucose using light energy absorbed by pigments during photosynthesis.

(a) Write the balanced chemical equation for photosynthesis. Include the essential conditions required above and below the reaction arrow. [3 marks]

(b) Describe and explain how the cellular structure and organization of the palisade mesophyll layer of a leaf is adapted to maximize the rate of photosynthesis. [3.8 marks]

(c) Explain the role of chlorophyll in photosynthesis in terms of energy transfer. [2 marks]

A student investigates the trends in Group 1 and Group 7 of the Periodic Table.

(a) Rubidium is a Group 1 alkali metal.
(i) State and explain the trend in reactivity of Group 1 alkali metals as the proton number increases. [3]
(ii) Predict the chemical equation for the reaction of rubidium with water. Include state symbols. [2]

(b) Chlorine is a Group 7 halogen.
(i) State the colour and physical state of chlorine at room temperature and pressure. [1]
(ii) Describe what is observed when chlorine gas is bubbled into a solution of potassium bromide. Explain this reaction in terms of reactivity. [3]

A marine survey vessel uses ultrasound to map the ocean floor.

(a) Ultrasound waves are longitudinal sound waves.
(i) Describe how a longitudinal wave transfers energy, referring to the movement of particles in the medium relative to the direction of wave travel. [2]
(ii) Explain the terms compression and rarefaction in relation to a longitudinal wave. [2]

(b) The vessel emits a pulse of ultrasound with a frequency of \(4.0 \times 10^4\text{ Hz}\). The wave travels through seawater at a speed of \(1500\text{ m/s}\).
(i) Calculate the wavelength of this ultrasound wave in seawater. State the formula used and show your working. [2.5]
(ii) The reflection of the ultrasound pulse from the seabed is detected \(1.2\text{ s}\) after transmission. Calculate the depth of the sea at this point. Show your working. [2.5]

Salivary amylase is an enzyme that catalyses the hydrolysis of starch into maltose.

(a) Outline the 'lock-and-key' hypothesis to explain how enzymes catalyse specific reactions. Refer to 'active site' and 'substrate' in your answer. [3]

(b) An investigation was carried out to study the effect of pH on the rate of starch breakdown by salivary amylase.
(i) State the reagent used to test for the presence of starch, and describe the colour change observed if starch is present. [2]
(ii) The reaction rate was found to be zero at pH 2. Explain why the amylase enzyme does not function at this pH. Use the terms denatured, active site, and complementary in your explanation. [4]

### Investigating the Effect of pH on Amylase Activity

A student investigates how pH affects the rate of starch breakdown by the enzyme amylase.

The student sets up four test tubes, each containing a mixture of starch and amylase solutions maintained at a specific pH using buffer solutions: pH 5.0, pH 6.0, pH 7.0, and pH 8.0.

Every 30 seconds, a drop of the reaction mixture is removed and added to one drop of iodine solution on a spotting tile. The time taken for the starch to be completely digested (when the iodine solution remains orange-brown instead of turning blue-black) is recorded.

Table 1.1 shows the colour observed at each time interval.

**Table 1.1**

| Time / s | pH 5.0 colour | pH 6.0 colour | pH 7.0 colour | pH 8.0 colour |
| :--- | :--- | :--- | :--- | :--- |
| 30 | blue-black | blue-black | blue-black | blue-black |
| 60 | blue-black | blue-black | orange-brown | blue-black |
| 90 | blue-black | orange-brown | orange-brown | blue-black |
| 120 | blue-black | orange-brown | orange-brown | blue-black |
| 150 | blue-black | orange-brown | orange-brown | blue-black |
| 180 | orange-brown | orange-brown | orange-brown | blue-black |

(a) (i) Using the data in Table 1.1, state the time taken for starch to be completely digested at:
* pH 6.0
* pH 7.0 [2]

(ii) Identify the independent variable in this investigation. [1]

(iii) Name two variables that must be kept constant to ensure a fair test. [2]

(iv) Before mixing, the amylase and starch solutions were incubated in a water bath at 35 °C for 5 minutes. Explain why this pre-incubation step is important. [1]

(v) Describe the chemical test used to confirm that reducing sugars are produced by the breakdown of starch, including the reagent used, the required reaction condition, and the positive result observed. [3]

(vi) Suggest how the student could modify the experiment to find a more precise value for the optimum pH. [1]

### Identification of Ions in Salt X

A student is provided with a green crystalline solid, salt X. They dissolve the solid in distilled water to make an aqueous solution, solution X, and perform qualitative tests to identify the cation and anion present.

**Test 1:** To a 2 cm³ portion of solution X, aqueous sodium hydroxide is added dropwise until in excess.
* **Observation:** A green precipitate is formed, which is insoluble in excess.

**Test 2:** To a 2 cm³ portion of solution X, aqueous ammonia is added dropwise until in excess.
* **Observation:** A green precipitate is formed, which is insoluble in excess.

**Test 3:** To a 2 cm³ portion of solution X, dilute nitric acid is added, followed by a few drops of aqueous barium nitrate.
* **Observation:** A white precipitate is formed.

(a) (i) Identify the cation present in salt X. [1]

(ii) Identify the anion present in salt X. [1]

(iii) Write the ionic equation, including state symbols, for the reaction occurring in Test 3. [2]

(b) The student wants to obtain pure, dry crystals of salt X from solution X. Describe the experimental steps of crystallization the student should perform. [4]

(c) State one hazard and one corresponding safety precaution for performing Test 3. [2]

### Investigating Thermal Insulators

A student investigates the cooling rates of hot water in three beakers under different conditions to determine which material is the best thermal insulator.

* **Beaker A:** Uninsulated beaker (control)
* **Beaker B:** Wrapped in one layer of bubble wrap
* **Beaker C:** Wrapped in one layer of cotton wool

The student pours 150 cm³ of hot water into each beaker, inserts a thermometer, and records the temperature every 60 seconds for 5 minutes.

Table 3.1 shows the temperature readings obtained.

**Table 3.1**

| Time / s | Beaker A Temperature / °C | Beaker B Temperature / °C | Beaker C Temperature / °C |
| :--- | :--- | :--- | :--- |
| 0 | 85.0 | 85.0 | 85.0 |
| 60 | 78.5 | 81.5 | 80.0 |
| 120 | 73.0 | 78.5 | 76.0 |
| 180 | 68.5 | 76.0 | 72.5 |
| 240 | 64.5 | 74.0 | 69.5 |
| 300 | 61.0 | 72.0 | 67.0 |

(a) (i) Suggest why the student waits for 30 seconds after pouring the hot water before taking the first reading at 0 s. [1]

(ii) State one precaution the student should take when reading the thermometer to ensure the measurement is accurate. [1]

(b) (i) Calculate the temperature drop (\(\Delta T\)) for the water in each beaker over the 300-second period.
* Beaker A: ____ °C
* Beaker B: ____ °C
* Beaker C: ____ °C [3]

(ii) Identify which material is the best thermal insulator. Justify your answer using the calculated temperature drops. [2]

(c) State two variables that must be kept constant to ensure a fair test when comparing Beaker B and Beaker C. [2]

(d) Suggest one modification to the apparatus that would reduce heat loss from the top of the beakers. [1]

A student investigates the effect of temperature on the rate of the breakdown of starch by the enzyme amylase.

The student mixes amylase and starch solutions together in a test-tube. At 30-second intervals, they remove a sample of the mixture and add it to a drop of iodine solution in a spotting tile.

(a) A thermometer is used to measure the temperature of the water bath. The major scale divisions are every \(10\ ^\circ\text{C}\) and the minor scale divisions are every \(1\ ^\circ\text{C}\). The top of the liquid column is exactly halfway between the \(37\ ^\circ\text{C}\) and \(38\ ^\circ\text{C}\) marks. State this temperature.

(b) State the color of the iodine solution when the starch has been completely broken down.

(c) The student records the following observations from the spotting tiles:
- At \(20\ ^\circ\text{C}\): the mixture turns blue-black at 0.5, 1.0, 1.5, and 2.0 minutes. It remains orange-brown at 2.5 minutes and all subsequent times.
- At \(40\ ^\circ\text{C}\): the mixture turns blue-black at 0.5 minutes. It remains orange-brown at 1.0 minute and all subsequent times.

State the time taken for starch to be completely broken down at \(20\ ^\circ\text{C}\) and at \(40\ ^\circ\text{C}\).

(d) Explain why the student used a thermostatically controlled water bath rather than heating the test-tubes directly with a Bunsen burner flame.

(e) State two variables, other than temperature, that must be kept constant to ensure a fair test.

(f) The student concludes that the optimum temperature for this amylase is \(40\ ^\circ\text{C}\). Suggest how the student could improve their experimental method to obtain a more precise value for the optimum temperature.

(g) Describe how the student can test the amylase solution to confirm that it does not contain any reducing sugars before starting the experiment. State the reagent used and the expected observation for a negative result.