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During a change of state for a pure substance, energy is used to overcome the intermolecular forces rather than raising the temperature. Therefore, the temperature of pure water remains constant at its boiling point of 100 degrees Celsius until the liquid has completely turned to gas.

1 mark for selecting the correct option C.

Electrons are negatively charged subatomic particles that orbit the nucleus. They have a relative charge of -1. Protons have a relative charge of +1, and neutrons are neutral with a relative charge of 0.

1 mark for selecting the correct option C.

The relative formula mass is calculated by adding the relative atomic masses of all atoms in the formula: \(M_r = 40 + 2 \times (16 + 1) = 40 + 2 \times 17 = 40 + 34 = 74\).

1 mark for selecting the correct calculation of 74 (C).

Methyl orange indicator is red in strongly acidic solutions (pH less than 3.1) and yellow in alkaline or neutral solutions.

1 mark for selecting the correct color red (C).

During the electrolysis of molten zinc chloride, positive zinc ions move towards the negative electrode (cathode) where they gain electrons to form zinc atoms (zinc metal). Negative chloride ions move to the positive electrode to form chlorine gas.

1 mark for selecting the correct product zinc metal (B).

Carbon is higher than copper in the reactivity series. Because carbon is more reactive than copper, it can reduce copper oxide by removing its oxygen.

1 mark for selecting the correct explanation (A).

The Rf value is calculated by dividing the distance moved by the spot by the distance moved by the solvent: \(R_f = \frac{6.0\text{ cm}}{8.0\text{ cm}} = 0.75\).

1 mark for selecting the correct Rf calculation of 0.75 (B).

Sodium chloride is an ionic compound with a high melting point. In the solid state, its ions are locked in fixed positions and cannot move to carry charge. However, when molten or dissolved in water, the ions become free to move, allowing it to conduct electricity.

1 mark for selecting the correct property (C).

Fractional distillation is the most suitable method to separate miscible liquids with different boiling points. The mixture is heated, and because ethanol has a lower boiling point (\(78^\circ\text{C}\)) than water (\(100^\circ\text{C}\)), it vaporises first, rises up the fractionating column, condenses in the Liebig condenser, and is collected as a pure liquid.

1 mark for correct option B.
- Reject any other option.

Hydrochloric acid is a strong acid. Strong acids have low pH values (between pH 1 and 2) and turn universal indicator red. pH 7 is neutral (green), pH 9 is a weak alkali (blue), and pH 14 is a strong alkali (purple).

1 mark for correct option A.
- Reject any other option.

In Dalton's model, atoms were thought to be indivisible, solid spheres with no internal structure. The modern model of the atom contains a nucleus containing protons and neutrons, surrounded by electrons in shells.

Award 1.5 marks for describing Dalton's model (indivisible solid sphere with no subatomic particles). Award 1 mark for describing the modern model in comparison (contains subatomic particles / contains a nucleus / has electrons).

To obtain pure, dry sand: 1. Filter the mixture to retrieve the sand as the residue on the filter paper. 2. Wash the sand with distilled water to remove any residual sodium chloride solution. 3. Dry the sand in a warm oven or leave it to dry.

Award 1.5 marks for filtration (mentioning filter paper / funnel / obtaining residue). Award 1 mark for washing with distilled water and drying.

First, place a drop of the unknown acid solution onto the universal indicator paper. Observe the color change of the paper. Finally, compare the color of the paper to a standard pH color chart to determine the corresponding pH value.

Award 1 mark for applying the solution to the indicator paper. Award 0.5 marks for observing the color change. Award 1 mark for comparing the color to a pH color chart.

Bioleaching uses bacteria to break down low-grade copper ores. The bacteria produce a solution called a leachate which contains dissolved copper compounds. Copper is then recovered from this leachate solution, typically by displacement with scrap iron or by electrolysis.

Award 1 mark for mentioning the use of bacteria to break down the ore. Award 1 mark for stating that a leachate solution containing copper ions/compounds is produced. Award 0.5 marks for explaining that copper is extracted from the leachate by displacement or electrolysis.

Calcium hydroxide has the formula \(\text{Ca(OH)}_2\). This contains 1 calcium atom, 2 oxygen atoms, and 2 hydrogen atoms. Calculation: \(40 + 2 \times (16 + 1) = 40 + (2 \times 17) = 40 + 34 = 74\).

Award 1.5 marks for correct calculation setup / substitution: \(40 + 2 \times (16 + 1)\) or equivalent. Award 1 mark for the correct final answer: 74.

The reaction between dilute hydrochloric acid and copper carbonate yields a salt (copper chloride), carbon dioxide gas, and water. Word equation: \(\text{hydrochloric acid} + \text{copper carbonate} \rightarrow \text{copper chloride} + \text{carbon dioxide} + \text{water}\)

Award 1.5 marks for correct reactants (hydrochloric acid + copper carbonate) and producing copper chloride. Award 1 mark for carbon dioxide + water.

Sodium is in Group 1 and has 1 outer electron, which it loses to obtain a stable full outer shell, forming a positive sodium ion (\(\text{Na}^+\)). Chlorine is in Group 7 and has 7 outer electrons; it gains the electron lost by sodium to form a negative chloride ion (\(\text{Cl}^-\)). These oppositely charged ions attract each other to form sodium chloride.

Award 1 mark for sodium losing one electron. Award 1 mark for chlorine gaining one electron. Award 0.5 marks for stating that oppositely charged ions are formed / attracted to each other.

The formula for calculating the \(R_f\) value is: \(R_f = \frac{\text{distance travelled by substance}}{\text{distance travelled by solvent front}}\). Substituting the given values: \(R_f = \frac{4.5\text{ cm}}{6.0\text{ cm}} = 0.75\).

Award 1.5 marks for the correct substitution in the formula: \(\frac{4.5}{6.0}\). Award 1 mark for the correct final answer: 0.75 (no units).

During simple distillation, the salty water is first heated so that the water evaporates or boils to form steam (liquid to gas). This steam then enters a condenser where it cools and condenses back into liquid water (gas to liquid), leaving the salt behind in the flask.

1 mark for identifying evaporation/boiling. 1 mark for identifying condensation. 0.5 marks for stating them in the correct sequence (evaporation first, then condensation).

The formula \(\text{Al}_2(\text{SO}_4)_3\) has 2 aluminium atoms: \(2 \times 27 = 54\), 3 sulfur atoms: \(3 \times 32 = 96\), and 12 oxygen atoms: \(12 \times 16 = 192\). Summing these together: \(M_r = 54 + 96 + 192 = 342\).

1 mark for correctly determining the number of each type of atom in the formula (2 Al, 3 S, 12 O). 1 mark for calculating the correct mass contribution of each element. 0.5 marks for the correct final sum of 342.

A neutral solution has a pH value of exactly 7. Phenolphthalein is an indicator that is pink in alkaline solutions and colorless in neutral and acidic solutions.

1 mark for stating pH 7. 1 mark for stating that phenolphthalein is colorless. 0.5 marks for linking both answers clearly.

Gold is an extremely unreactive metal, so it does not react with other elements like oxygen in the environment, remaining as a pure element (native metal). Iron is a moderately reactive metal, meaning it readily reacts with oxygen and water in the environment to form iron oxides (ores).

1 mark for stating that gold is very unreactive. 1 mark for stating that iron is more reactive / reacts with oxygen. 0.5 marks for linking their reactivity to why they exist as native metals vs compounds.

Calcium is in Group 2, so it has 2 outer shell electrons. It transfers these 2 outer electrons to an oxygen atom, which is in Group 6 and needs 2 electrons to complete its outer shell. This results in a calcium ion with a \(2+\) charge (\(\text{Ca}^{2+}\)) and an oxide ion with a \(2-\) charge (\(\text{O}^{2-}\)).

1 mark for describing the transfer of 2 electrons from calcium to oxygen. 1 mark for stating the correct charges (\(2+\) for calcium and \(2-\) for oxide). 0.5 marks for specifying that ions with full outer shells are formed.

Molten lead bromide contains lead ions (\(\text{Pb}^{2+}\)) and bromide ions (\(\text{Br}^-\)). The positive lead ions move to the negative cathode, where they gain electrons to form lead metal. The negative bromide ions move to the positive anode, where they lose electrons to form bromine gas.

1 mark for identifying lead is formed at the cathode. 1 mark for identifying bromine/bromine gas is formed at the anode. 0.5 marks for correctly matching both products to the correct electrodes.

The formula for calculating the \(R_f\) value is \(R_f = \text{distance travelled by substance} / \text{distance travelled by solvent}\). Substituting the given values: \(R_f = 3.2\text{ cm} / 8.0\text{ cm} = 0.4\).

1 mark for showing the correct formula for \(R_f\). 1 mark for dividing 3.2 by 8.0. 0.5 marks for the correct final answer of 0.4 (no units).

To test for hydrogen gas, collect some of the gas in a test tube and apply a lighted wooden splint. If hydrogen is present, it will burn with a characteristic 'squeaky pop' sound.

1 mark for introducing a lighted/burning splint (reject glowing splint). 1 mark for stating it makes a 'squeaky pop' sound. 0.5 marks for clear and logical description of the test procedure.

The atomic number represents the number of protons in an atom's nucleus, which is 5. The mass number is the sum of protons and neutrons, which is 11. Therefore, the number of neutrons is calculated by subtracting the atomic number from the mass number: \(11 - 5 = 6\).

1 mark for identifying 5 protons. 1.5 marks for calculating 6 neutrons (award 1 mark for the working \(11 - 5\) even if calculation is incorrect).

Filtration is used to separate an insoluble solid (sand) from a liquid or solution (sodium chloride solution). During filtration, the solid left behind on the filter paper is called the residue.

1 mark for naming 'filtration' (or 'filtering'). 1.5 marks for identifying the solid left on the paper as the 'residue'.

Adding an alkali to an acidic solution reduces the concentration of hydrogen ions, which causes the pH of the solution to increase (moving towards 7 and eventually above). The reaction between an acid and an alkali is called a neutralisation reaction.

1.5 marks for stating that the pH increases (accept: pH goes up / becomes less acidic). 1 mark for identifying the reaction as 'neutralisation'.

Carbon is higher in the reactivity series than copper, meaning carbon can reduce copper oxide by removing its oxygen (displacement). However, aluminium is more reactive than carbon, so carbon is not strong enough to displace or reduce aluminium from aluminium oxide.

1.5 marks for explaining that carbon is more reactive than copper (so carbon can displace/reduce copper). 1 mark for explaining that carbon is less reactive than aluminium (so carbon cannot displace/reduce aluminium).

The formula for calcium hydroxide is \(\text{Ca(OH)}_2\). This contains:
- 1 \(\times\) Ca atom = \(1 \times 40 = 40\)
- 2 \(\times\) O atoms = \(2 \times 16 = 32\)
- 2 \(\times\) H atoms = \(2 \times 1 = 2\)

Adding these values together:
\(M_r = 40 + 32 + 2 = 74\).

1.5 marks for showing the correct working: \(40 + 2 \times (16 + 1)\) or equivalent. 1 mark for the correct final relative formula mass of 74.

Fractional distillation works because different liquids have different boiling points. When the mixture is heated, the liquid with the lower boiling point (ethanol) evaporates first. Its vapour rises up the fractionating column, enters the condenser where it is cooled back into a liquid, and is collected separately.

1 mark for stating that the liquids have different boiling points. 1.5 marks for describing how the one with the lower boiling point evaporates first and is then cooled/condensed.

- At the positive electrode (anode), negative bromide ions (\(\text{Br}^-\)) lose electrons to form bromine gas (\(\text{Br}_2\)), which is seen as brown fumes.
- At the negative electrode (cathode), positive lead ions (\(\text{Pb}^{2+}\)) gain electrons to form lead metal (\(\text{Pb}\)), which is observed as a grey liquid or shiny grey deposit.

1.25 marks for describing the observation at the anode as brown gas/fumes (accept: orange-brown gas). 1.25 marks for describing the observation at the cathode as grey metal/liquid/solid.

Phytomining involves growing plants on soil containing low-grade copper ores. The plants absorb copper compounds through their roots and concentrate them in their tissues. The plants are then harvested and burned to produce ash. The ash, which contains a high concentration of copper compounds, can then be processed (e.g., reacted with acid to form a leachate) to extract the copper.

1 mark for stating that plants are grown on low-grade ores to absorb metal/copper compounds. 1 mark for stating that plants are burned to produce ash. 0.5 marks for stating that the ash is processed to obtain the copper.

The relative formula mass is calculated by adding the relative atomic masses of all the atoms in the formula: \(M_r = (2 \times 56) + (3 \times 16) = 112 + 48 = 160\).

- 1.5 marks for showing correct working: \((2 \times 56) + (3 \times 16)\)
- 1.0 mark for the correct final answer of 160

Filtration works because the large insoluble sand particles cannot pass through the tiny pores in the filter paper, whereas the water molecules can easily pass through. The sand is left behind as the residue, and the water is collected as the filtrate.

- 1.0 mark for stating that the mixture is poured through filter paper in a funnel
- 1.0 mark for explaining that the sand (residue) is trapped by the filter paper
- 0.5 marks for stating that the water (filtrate) passes through the paper

Each change of 1 on the pH scale represents a 10-fold change in hydrogen ion concentration. When an acid is diluted by a factor of 10, the concentration of \(\text{H}^+\) ions decreases by a factor of 10, so the pH increases by 1. The new pH is \(2.0 + 1 = 3.0\).

- 1.0 mark for the correct new pH of 3 (or 3.0)
- 1.5 marks for explaining that diluting by a factor of 10 decreases the \(\text{H}^+\) ion concentration 10-fold, which increases the pH by 1

Zinc is more reactive than copper, so a displacement reaction occurs. The blue color of the copper sulfate solution fades (or turns colorless), and a reddish-brown solid of copper metal is deposited. Copper ions (\(\text{Cu}^{2+}\)) gain electrons to form copper atoms, which means copper ions are reduced.

- 1.5 marks for a correct observation (e.g., blue solution fades/turns colorless OR reddish-brown solid forms)
- 1.0 mark for identifying that copper ions (or copper) are reduced

The atomic number of carbon is 6, which represents the number of protons. Since the atom is neutral, there are also 6 electrons. The mass number is 14, so the number of neutrons is \(14 - 6 = 8\) neutrons.

- 1.0 mark for identifying 6 protons and 6 electrons
- 1.5 marks for calculating and identifying 8 neutrons

The \(R_f\) value is calculated using the formula: \(R_f = \frac{\text{distance travelled by the substance}}{\text{distance travelled by the solvent front}} = \frac{4.5}{6.0} = 0.75\).

- 1.5 marks for showing the correct formula or substitution: \(\frac{4.5}{6.0}\)
- 1.0 mark for the correct calculated \(R_f\) value of 0.75 (no units required)

During the electrolysis of molten lead bromide, negative bromide ions (\(\text{Br}^-\)) are attracted to the positive anode, where they lose electrons to form bromine molecules (\(\text{Br}_2\)). This is observed as orange-brown fumes of bromine gas.

- 1.0 mark for identifying the product as bromine
- 1.5 marks for describing the correct observation of orange-brown fumes or gas

In phytoextraction, plants are grown on soil containing low-grade metal ores. The plants absorb metal ions through their roots and concentrate them. The plants are then harvested and burned to produce ash, which contains a high concentration of the metal compounds, from which the metal can be extracted.

- 1.0 mark for stating that plants absorb metal compounds or ions from the soil
- 1.5 marks for stating that the plants are harvested and burned to produce ash

The mixture of sand, salt, and water can be separated using filtration followed by crystallisation. First, set up a funnel with filter paper placed over a conical flask. Pour the mixture into the filter paper. The sand is insoluble in water and its particles are too large to pass through the filter paper, so it remains on the paper as the residue. The sodium chloride is soluble and dissolved in the water, so the salt solution passes through the paper as the filtrate. Rinse the sand residue with a small amount of distilled water to wash away any remaining salt solution, then leave the sand in a warm oven or on a windowsill to dry. Next, pour the filtrate into an evaporating basin. Heat the solution using a Bunsen burner to evaporate most of the water. Stop heating when crystals start to form at the edges of the basin. Leave the remaining concentrated solution to cool slowly so that large, well-defined sodium chloride crystals form. Finally, filter the crystals from any remaining liquid and pat them dry using filter paper.

Level 1 (1-2 marks): The response identifies at least one correct separation technique (filtration or evaporation) but lacks detail on how to perform it or why it works. Level 2 (3-4 marks): The response describes both filtration and evaporation/crystallisation in a logical order. Some scientific reasons are provided (e.g., sand is insoluble so it is filtered; water evaporates to leave salt). Level 3 (5-6 marks): The response provides a detailed, logical step-by-step method for both filtration and crystallisation. Complete explanations are given for why sand is separated (insolubility/particle size) and how dry, pure crystals of salt are obtained (heating to crystallisation point, slow cooling, and drying).

To investigate the change in pH, first measure a fixed volume, such as 25 cubic centimetres, of dilute hydrochloric acid using a measuring cylinder and pour it into a beaker. Measure the initial pH of the acid using a pH probe connected to a pH meter, or by using universal indicator paper and comparing the colour to a standard chart, and record this value. Weigh a 0.3g portion of calcium hydroxide powder on a balance. Add this portion of calcium hydroxide to the beaker containing the acid and stir thoroughly with a glass rod to ensure the reaction goes to completion. Measure and record the new pH of the mixture. Repeat the process of weighing and adding 0.3g portions of calcium hydroxide, stirring, and recording the pH after each addition. Continue this process until the pH stops changing, which indicates that the acid has been completely neutralised and the calcium hydroxide is now in excess. To show the change in pH, plot a graph of pH on the y-axis against the total mass of calcium hydroxide added on the x-axis.

Level 1 (1-2 marks): The response mentions adding calcium hydroxide to acid and measuring pH but lacks a clear sequence or specific measurements of quantities. Level 2 (3-4 marks): The response describes a structured method that includes measuring a volume of acid, adding known, repeated masses of calcium hydroxide, and measuring pH. The steps are mostly logical but may omit details like stirring, measuring initial pH, or plotting the graph. Level 3 (5-6 marks): The response provides a detailed, step-by-step method including: measuring a specific volume of acid, measuring initial pH, adding fixed small masses of calcium hydroxide sequentially, stirring after each addition, measuring pH after each addition, continuing until the pH is constant (excess), and explaining how to use the data by plotting a graph of pH against total mass.

Potassium is a highly reactive Group 1 metal. When added to water, it floats, melts into a shiny ball because of the heat produced, moves quickly across the surface, and reacts vigorously with water to produce hydrogen gas, which ignites with a characteristic lilac flame.

1 mark for selecting the correct option (b). All other options score 0.

The relative formula mass of \(\text{Ca(OH)}_2\) is calculated by adding the relative atomic masses of all the constituent atoms: \(M_r = 40 + 2 \times (16 + 1) = 40 + 2 \times 17 = 40 + 34 = 74\).

1 mark for selecting the correct option (c). All other options score 0.

The fractionating column is hottest at the bottom and coolest at the top. The fractions with the highest boiling points condense at the bottom, and these fractions consist of the largest hydrocarbon molecules with the strongest intermolecular forces.

1 mark for selecting the correct option (b). All other options score 0.

Increasing temperature increases the kinetic energy of the particles, causing them to move faster and collide more frequently. Additionally, a much higher proportion of colliding particles now possess energy equal to or greater than the activation energy, leading to a higher frequency of successful collisions.

1 mark for selecting the correct option (c). All other options score 0.

Chlorine is more reactive than iodine because it is higher up in Group 7. It displaces iodine from potassium iodide solution, forming potassium chloride and iodine. The dissolved iodine causes the solution to turn brown.

1 mark for selecting the correct option (a). All other options score 0.

The early atmosphere was composed mainly of carbon dioxide and water vapour released by volcanic activity, with little to no oxygen. Over billions of years, carbon dioxide dissolved in oceans and was taken in by plants for photosynthesis, reducing its abundance to around 0.04% today.

1 mark for selecting the correct option (c). All other options score 0.

A catalyst increases the rate of a chemical reaction by providing an alternative reaction pathway that has a lower activation energy, meaning more colliding particles have sufficient energy to successfully react.

1 mark for selecting the correct option (a). All other options score 0.

A brick-red (or orange-red) flame in a flame test is the characteristic test result used to identify calcium ions (\(\text{Ca}^{2+}\)). Lithium gives a red flame, sodium gives yellow, and copper gives green-blue.

1 mark for selecting the correct option (c). All other options score 0.

Chlorine gas is acidic and is a strong bleaching agent. When damp blue litmus paper is exposed to chlorine gas, it first turns red because the chlorine dissolves in the water on the paper to form an acidic solution, and then it is bleached white.

B is the correct option (1 mark). Any other option scores 0 marks.

Two ways to increase the rate of reaction are:
1. Increase the concentration of the hydrochloric acid.
2. Increase the surface area of the calcium carbonate (e.g., using powder instead of marble chips).

Explanation using collision theory:
- If concentration is increased: There are more reactant particles in the same volume. This leads to more frequent successful collisions.
- If surface area is increased: More reactant particles are exposed to the acid. This leads to more frequent successful collisions.

- Award 0.5 marks for each correct method (maximum of 1 mark for two methods chosen from: increase concentration, increase surface area / use powder).
- Award 1 mark for describing the physical change in particles (e.g., more particles in a given volume for concentration, or more particles exposed for surface area).
- Award 0.5 marks for linking this to more frequent collisions / higher frequency of collisions.

Argon is a Group 0 noble gas. Its electronic configuration is 2,8,8. Since its outer electron shell is completely full (containing 8 electrons), it has a stable electronic structure. Because of this stability, argon has no tendency to gain, lose, or share electrons to form chemical bonds, making it extremely unreactive. An everyday use of argon is in light bulbs to provide an inert atmosphere that prevents the hot tungsten filament from burning away.

- Award 1 mark for stating that argon has a full outer electron shell / stable electronic configuration.
- Award 1 mark for explaining that it does not lose, gain, or share electrons / does not form chemical bonds.
- Award 0.5 marks for a valid use (e.g., in light bulbs, food packaging, or welding).

Sulfur dioxide is formed because fossil fuels contain sulfur as an impurity. When these fuels are burned (combustion), the sulfur reacts with oxygen in the air to produce sulfur dioxide gas:
\( \text{S} + \text{O}_2 \rightarrow \text{SO}_2 \).
Once released into the atmosphere, sulfur dioxide dissolves in water droplets in clouds, forming dilute sulfuric acid, which falls as acid rain. Acid rain damages buildings, kills aquatic life, and harms trees.

- Award 1 mark for stating that fossil fuels contain sulfur impurities.
- Award 1 mark for stating that sulfur reacts with oxygen (is oxidized) during combustion.
- Award 0.5 marks for identifying acid rain as the environmental problem.

The Earth receives electromagnetic radiation from the Sun. Some of this radiation is absorbed by the Earth, warming it up, and is then re-radiated back into space as longer-wavelength infrared radiation. Greenhouse gases in the atmosphere absorb this infrared radiation and re-emit it in all directions, including back towards the Earth. This process traps heat and keeps the planet warm enough to support life. Another common greenhouse gas is methane (or water vapour).

- Award 1 mark for stating that greenhouse gases absorb heat/infrared radiation emitted or re-radiated from the Earth's surface.
- Award 1 mark for explaining that they re-emit this radiation back towards Earth / trap heat in the atmosphere.
- Award 0.5 marks for naming another greenhouse gas (e.g., methane / water vapour / nitrous oxide).

To calculate the relative formula mass of \(\text{Ca(OH)}_2\):
1. Identify the number of atoms of each element in the formula:
- 1 calcium atom (\(\text{Ca}\))
- 2 oxygen atoms (\(\text{O}\))
- 2 hydrogen atoms (\(\text{H}\))

2. Multiply the number of atoms by their respective relative atomic masses (\(A_r\)):
- \(\text{Ca}: 1 \times 40 = 40\)
- \(\text{O}: 2 \times 16 = 32\)
- \(\text{H}: 2 \times 1 = 2\)

3. Add these values together:
\(M_r = 40 + 32 + 2 = 74\).

- Award 0.5 marks for correctly identifying the atom counts (1 Ca, 2 O, 2 H) or writing \(40 + 2 \times (16 + 1)\).
- Award 1 mark for correct substitution of values: \(40 + (2 \times 16) + (2 \times 1)\) or equivalent.
- Award 1 mark for the correct final answer of 74.

In an exothermic reaction, heat energy is released to the surroundings. Therefore, the total energy of the products is lower than the total energy of the reactants. As heat energy is released, the temperature of the surroundings increases. The activation energy is defined as the minimum amount of energy that reacting particles must have to collide successfully and start a chemical reaction.

- Award 1 mark for stating that the energy of the products is lower than the energy of the reactants.
- Award 0.5 marks for stating that the temperature of the surroundings increases.
- Award 1 mark for defining activation energy as the minimum energy required to start a reaction / for particles to react.

Chlorine is more reactive than bromine because it is higher up in Group 7. Therefore, chlorine will displace bromine from sodium bromide. This is a displacement reaction.
The word equation is:
chlorine + sodium bromide \(\rightarrow\) sodium chloride + bromine

The starting solution of sodium bromide is colourless. When bromine is displaced and dissolves in the water, it turns the solution orange (or yellow/brown).

- Award 0.5 marks for identifying the reaction as a displacement reaction.
- Award 1 mark for the correct word equation: chlorine + sodium bromide \(\rightarrow\) sodium chloride + bromine (accept correct formulas \(\text{Cl}_2 + 2\text{NaBr} \rightarrow 2\text{NaCl} + \text{Br}_2\)).
- Award 1 mark for stating that the solution turns orange / yellow / brown (from colourless).

Crude oil is first heated until it vaporises and is passed into the fractionating column. The column has a temperature gradient: it is very hot at the bottom and becomes progressively cooler towards the top. As the vapours rise up the column, they cool. When they reach a level where the temperature is lower than their specific boiling point, they condense back into a liquid and are piped off. Hydrocarbons with high boiling points condense near the bottom, while those with lower boiling points rise further up and condense near the top.

- Award 1 mark for stating that the column has a temperature gradient / is hot at the bottom and cool at the top.
- Award 1 mark for explaining that fractions condense back into liquids at different heights depending on their boiling points.
- Award 0.5 marks for specifying that larger hydrocarbons with high boiling points condense at the bottom, OR smaller hydrocarbons with lower boiling points condense at the top.

During the reaction between sodium and water, hydrogen gas is produced which causes effervescence (fizzing) and makes the sodium float and move rapidly on the surface. The heat of the reaction melts the sodium into a shiny sphere. Sodium hydroxide, a strong alkali, is formed in the solution, causing the pH of the water to increase (become alkaline).

Award 1 mark for any correct observation (such as: melts into a ball, moves on the surface, fizzes/bubbles/effervescence, trail of bubbles, or ignites with a yellow/orange flame) up to a maximum of 2 observations (2 marks total). Award 0.5 marks for stating that the pH increases or the solution becomes alkaline.

The formula of calcium hydroxide is \(\text{Ca(OH)}_2\). This contains 1 calcium atom, 2 oxygen atoms, and 2 hydrogen atoms. \(M_r = 40 + (2 \times 16) + (2 \times 1) = 40 + 32 + 2 = 74\).

Award 1.5 marks for showing correct working (either showing the addition of all atoms or substituting the values correctly: \(40 + 2 \times (16 + 1)\)). Award 1 mark for the correct final answer of 74.

The temperature decreased from \(20.2^\circ\text{C}\) to \(11.8^\circ\text{C}\). A decrease in temperature indicates that the reaction system absorbed thermal energy from its surroundings, which is the definition of an endothermic reaction.

Award 1 mark for identifying the reaction as endothermic. Award 1 mark for stating that the temperature decreased. Award 0.5 marks for explaining that thermal energy is taken in/absorbed from the surroundings.

When coal is burned, the sulfur impurities react with oxygen from the air to form sulfur dioxide gas (\(\text{SO}_2\)). This gas escapes into the atmosphere, where it dissolves in water vapor in clouds to form sulfurous/sulfuric acid, which falls as acid rain.

Award 1 mark for naming 'sulfur dioxide' as the gas responsible. Award 1 mark for explaining that sulfur reacts with oxygen when coal burns. Award 0.5 marks for stating that the sulfur dioxide dissolves in water/rainwater in the atmosphere to form acid rain.

Chlorine is more reactive than bromine, so it displaces bromide ions from potassium bromide. The reaction is: \(\text{Cl}_2 + 2\text{KBr} \rightarrow 2\text{KCl} + \text{Br}_2\). The formation of diatomic bromine (\(\text{Br}_2\)) dissolved in water causes the solution to change from colorless to an orange/yellow-brown color.

Award 1 mark for stating the solution turns orange, yellow-brown, or brown (reject red or yellow alone). Award 1 mark for naming 'bromine' (or formula \(\text{Br}_2\)) as the substance produced. Award 0.5 marks for explaining that chlorine displaces bromine because chlorine is more reactive than bromine.

Using powdered calcium carbonate instead of large marble chips increases the surface area exposed to the acid. This means more calcium carbonate particles are available to collide with the acid particles, resulting in a higher frequency of successful collisions (more collisions per second) and therefore a faster reaction rate.

Award 1 mark for stating that powder has a larger surface area (for the same mass). Award 1 mark for stating that there are more frequent collisions / more collisions per second. Award 0.5 marks for specifying that these collisions occur between reactant particles (calcium carbonate and acid particles).

Crude oil is separated using fractional distillation. The mixture is vaporized and enters a fractionating column which is hot at the bottom and cool at the top. Hydrocarbons have different boiling points because of their different chain lengths. They rise up the column and condense back into liquids at the heights where the temperature matches their boiling points.

Award 1 mark for naming 'fractional distillation'. Award 1 mark for stating that different hydrocarbons have different boiling points. Award 0.5 marks for explaining that they condense at different heights/temperatures in the fractionating column.

First, calculate the relative formula mass (\(M_r\)) of \(\text{CO}_2\): \(12 + (16 \times 2) = 44\). Next, calculate the total mass of oxygen in the formula: \(2 \times 16 = 32\). Finally, calculate the percentage by mass: \(\frac{32}{44} \times 100 = 72.7272...\%\). Rounded to 1 decimal place, this is \(72.7\%\).

Award 1 mark for calculating the \(M_r\) of \(\text{CO}_2\) as 44. Award 1 mark for showing the correct calculation expression: \(\frac{32}{44} \times 100\). Award 0.5 marks for the correct final answer of 72.7% (accept 72.7 without the percentage sign, but penalise if not rounded to 1 decimal place).

Sodium is a highly reactive Group 1 metal. When added to water, it reacts vigorously to form sodium hydroxide and hydrogen gas. Because sodium is less dense than water, it floats. The reaction is highly exothermic, which melts the sodium into a shiny ball. The production of hydrogen gas causes fizzing and propels the sodium across the water surface. Since sodium hydroxide is a strong alkali, the universal indicator turns purple (or blue).

1 mark: Any one correct observation (e.g. sodium floats / melts into a ball / fizzes / moves on surface / disappears). 1 mark: A second correct observation from the list. 0.5 marks: Stating the final colour is purple or blue.

An increase in temperature indicates that heat energy is being released from the chemical system into the surroundings. This is an exothermic reaction. In an exothermic reaction, the chemical energy of the reactants is higher than that of the products because energy is lost to the surroundings.

1 mark: Stating the reaction is exothermic. 1 mark: Stating the products have lower energy than the reactants (or reactants have higher energy than products). 0.5 marks: Stating that energy is released/transferred to the surroundings.

As the carbon chain length increases, the size of the hydrocarbon molecules increases. This leads to stronger intermolecular forces between the molecules, requiring more thermal energy to overcome, which increases the boiling point. Additionally, larger molecules become more easily tangled, increasing the viscosity (making them thicker and less runny).

1 mark: Boiling point increases. 1 mark: Viscosity increases (or becomes thicker/less runny). 0.5 marks: Explaining that larger molecules have stronger intermolecular forces or get tangled more easily.

The relative formula mass is calculated by adding the relative atomic masses of all atoms in the formula: \(M_r\) of \(\text{Ca(OH)}_2 = 40 + 2 \times (16 + 1) = 40 + 2 \times 17 = 40 + 34 = 74\).

1 mark: Correct calculation of the mass of the hydroxide group, \(2 \times (16 + 1) = 34\) (or showing \(40 + 16 + 16 + 1 + 1\)). 1.5 marks: Correct final answer of 74. (Full 2.5 marks awarded for correct final answer without working).

Short-wavelength radiation from the Sun passes through the Earth's atmosphere and warms the Earth's surface. The Earth's surface then re-radiates this energy as longer-wavelength infrared radiation (heat). Greenhouse gases in the atmosphere absorb this infrared radiation and re-emit it in all directions, including back towards the Earth, trapping the heat.

1 mark: Stating that the Earth's surface emits infrared / heat radiation. 1 mark: Stating that greenhouse gases absorb this infrared / heat radiation. 0.5 marks: Stating that the gases re-emit the radiation back to Earth / trap heat in the atmosphere.

Argon is a noble gas and belongs to Group 0 of the periodic table. It has a full outer shell of electrons, making it completely inert (unreactive). Filament light bulbs contain a metal filament that gets extremely hot. If oxygen from the air were present, the filament would burn and oxidize. Filling the bulb with argon prevents this reaction, extending the bulb's lifespan.

1 mark: Stating that argon is inert / unreactive. 1 mark: Explaining that it prevents the filament from burning / reacting with oxygen. 0.5 marks: Correctly identifying argon as being in Group 0 (or Group 8 / Noble Gases).

Powdered marble has a much larger surface area than the same mass of large marble chips. This means more calcium carbonate particles are exposed on the surface to collide with the acid particles. As a result, there are more frequent successful collisions per second, which increases the rate of reaction.

1 mark: Stating that the rate of reaction increases because powder has a larger surface area. 1 mark: Explaining that this leads to more frequent collisions / more collisions per second. 0.5 marks: Explicitly linking the increased collision frequency to the increased rate of reaction.

1. Calculate the moles of each element: Moles of \(\text{Cu} = \frac{12.7}{63.5} = 0.2\text{ mol}\). Moles of \(\text{O} = \frac{1.6}{16} = 0.1\text{ mol}\). 2. Find the simplest whole-number ratio: Divide both by the smallest number of moles (0.1): \(\text{Cu} = \frac{0.2}{0.1} = 2\), \(\text{O} = \frac{0.1}{0.1} = 1\). 3. Therefore, the empirical formula is \(\text{Cu}_2\text{O}\).

1 mark: Correct calculation of moles for both Cu (0.2) and O (0.1). 1 mark: Finding the simplest whole-number ratio of 2:1. 0.5 marks: Stating the correct empirical formula of \(\text{Cu}_2\text{O}\).

Potassium has more occupied electron shells than sodium, which means its outer shell electron is further from the positive nucleus. This results in a weaker electrostatic attraction between the nucleus and the outer electron, meaning less energy is needed to remove it and it is lost more easily in reactions.

1 mark for stating that potassium has more occupied electron shells (or the outer electron is further from the nucleus).
1 mark for stating that there is a weaker attraction between the nucleus and the outer electron.
0.5 marks for stating that the outer electron is lost more easily.

As the Earth cooled, water vapour in the early atmosphere condensed to form the oceans. Large quantities of carbon dioxide dissolved in these newly formed oceans. This dissolved carbon dioxide reacted to form insoluble metal carbonates, which precipitated out and formed sedimentary rocks, trapping the carbon.

1 mark for stating water vapour condensed to form oceans.
1 mark for stating carbon dioxide dissolved in the oceans.
0.5 marks for stating that carbonates precipitated or sedimentary rocks (like limestone) were formed.

Increasing the temperature gives the reactant particles more kinetic energy, causing them to move faster. Consequently, they collide more frequently. Furthermore, a greater proportion of these collisions have energy equal to or greater than the activation energy, resulting in a higher frequency of successful collisions.

1 mark for stating particles gain kinetic energy / move faster, leading to more frequent collisions.
1 mark for stating a higher proportion of collisions have energy equal to or greater than the activation energy.
0.5 marks for concluding this results in a higher rate of successful collisions.

To calculate the relative formula mass of \(\text{Na}_2\text{CO}_3\):
\(M_r = (2 \times 23) + 12 + (3 \times 16)\)
\(M_r = 46 + 12 + 48 = 106\)

1 mark for showing correct multiplication: \((2 \times 23)\) and \((3 \times 16)\).
1 mark for correctly adding the values together: \(46 + 12 + 48\).
0.5 marks for the correct final answer of 106.

Crude oil is heated and vaporised before entering the fractionating column, which is hot at the bottom and cool at the top. The vaporised hydrocarbons rise up the column and condense when they reach a level that matches their boiling point. Shorter-chain hydrocarbons with lower boiling points condense near the top, while longer-chain hydrocarbons with higher boiling points condense near the bottom.

1 mark for stating crude oil is vaporised before entering the column.
1 mark for stating the column is hot at the bottom and cool at the top (has a temperature gradient).
0.5 marks for stating that fractions condense at different heights/levels according to their boiling points.

To test for sulfate ions, dilute hydrochloric acid is first added to the sample to remove any carbonate impurities. Barium chloride solution is then added. If sulfate ions are present, a white precipitate of barium sulfate will form.

1 mark for adding dilute hydrochloric acid (accept dilute nitric acid; reject sulfuric acid).
1 mark for adding barium chloride solution (accept barium nitrate solution).
0.5 marks for identifying the positive observation of a white precipitate.

When chlorine water is added to potassium bromide solution, the colourless solution turns orange or yellow-brown. This color change occurs because chlorine is more reactive than bromine and displaces the bromide ions from the solution, forming elemental bromine: \(\text{Cl}_2 + 2\text{KBr} \rightarrow 2\text{KCl} + \text{Br}_2\).

1 mark for the observation: solution turns orange/yellow-brown.
1 mark for stating that chlorine is more reactive than bromine.
0.5 marks for stating that chlorine displaces bromine from the compound.

Safety precautions: Wear eye protection, use safety screens, handle the metals using forceps or tweezers, use small (rice-grain sized) pieces of metal, and use a large volume of water in a trough. Similarities: All three metals float on water, fizz/effervesce (producing hydrogen gas), move on the water surface, and eventually disappear to form an alkaline solution. Differences: Lithium reacts slowly and retains its shape. Sodium reacts more vigorously and melts into a shiny ball. Potassium reacts very violently, melts into a ball, and burns with a lilac-coloured flame. Explanation of trend: Reactivity increases down the group. As we go down Group 1, the atoms have more shells of electrons and therefore get larger. This means the single outer electron is further away from the positive nucleus, and there is more shielding from the inner electron shells. This weakens the electrostatic attraction between the positive nucleus and the outer electron, meaning the outer electron is lost more easily.

Level 1 (1-2 marks): Simple description of safety OR at least one observation OR states that reactivity increases down the group. The response lacks detail and structure. Level 2 (3-4 marks): Describes safety precautions and some observations for the metals, identifying some similarities or differences. Attempts to explain the reactivity trend (e.g. mentions losing an electron or atoms getting larger). Level 3 (5-6 marks): Explains safety precautions, provides detailed observations covering both similarities and differences, and gives a clear, structured explanation of the reactivity trend down the group in terms of atomic size, shielding, and the ease of losing the outer electron.

How the column works: Crude oil is heated and vaporised before entering the bottom of the fractionating column. There is a temperature gradient in the column, which is hot at the bottom and cooler at the top. The vapours rise up the column, cool, and condense back into liquids at different levels where the temperature falls below their boiling point. The different liquid fractions are then piped off. Properties of hydrocarbons: Smaller hydrocarbon molecules have lower boiling points, lower viscosity, and are more flammable; they condense and are collected near the top of the column. Larger molecules have higher boiling points, higher viscosity, and are less flammable; they condense and are collected near the bottom. Fractions and uses (any two): 1. Refinery gases - used for domestic heating and cooking. 2. Petrol - fuel for cars. 3. Kerosene - fuel for aircraft. 4. Diesel - fuel for trains and heavy vehicles. 5. Fuel oil - fuel for ships and power stations. 6. Bitumen - surfacing roads and roofs.

Level 1 (1-2 marks): Describes a basic aspect of fractional distillation (e.g. heating crude oil) or names a fraction with its use. There is little explanation of how the column works. Level 2 (3-4 marks): Describes the process including the temperature gradient and condensation. Explains how molecular size affects boiling point and condensation height. Names at least one fraction and its correct use. Level 3 (5-6 marks): Gives a comprehensive, logical explanation of the industrial process, including the temperature gradient and how boiling point determines where a fraction condenses. Correctly names at least two fractions and their respective uses.