Mark Scheme

Section A — Structured Questions

Question 1

(a) • Sodium ion shown as [Na]⁺ with no electrons shown (1) • Chloride ion shown as [Cl]⁻ with 8 electrons in outer shell (arranged in pairs or as taught) (1) Accept: electrons shown as dots, crosses, or a combination Reject: electrons not in pairs around Cl; incorrect charges

(b)(i) • Strong electrostatic forces / ionic bonds between oppositely charged ions (1) • A large amount of energy is needed to break these forces / bonds (1) Accept: attraction between ions Reject: "strong bonds" without specifying ionic or electrostatic

(b)(ii) • The regular lattice structure breaks down / ions become free to move (1) Accept: ions separate Reject: bonds break (incomplete)

(c) • The ions are free to move / become mobile (1) • (These mobile ions) carry the electric charge / current (1) Accept: delocalised ions Reject: electrons carry the charge

(d) • 250 cm³ = 0.25 dm³ (1) • Concentration = 5.85 ÷ 0.25 = 23.4 g/dm³ (1) Accept: 23 g/dm³ Award (1) for correct working even if arithmetic error in final answer

[Total: 9 marks]

Question 2

(a) • A: solid (1) • B: liquid (1) • C: gas (1)

(b)(i) • Energy increases / particles gain energy (1)

(b)(ii) • Particles become less ordered / move from regular to irregular arrangement (1) Accept: particles can move past each other

(c) • Particles move faster / rapidly / vigorously (1) Accept: particles vibrate more Reject: particles move freely (this is after boiling)

(d) • Sublimation (1)

[Total: 7 marks]

Question 3

(a) • Loss of oxygen (1)

(b) • Iron(III) oxide / Fe₂O₃ (is reduced) (1) • Because it loses oxygen / oxygen is removed from it (1) Accept: Fe₂O₃ for first mark

(c) • Molar mass of Fe₂O₃ = (56 × 2) + (16 × 3) = 160 (1) • Moles of Fe₂O₃ = 160 ÷ 160 = 1 mole • From equation: 1 mole Fe₂O₃ produces 2 moles Fe • Mass of Fe = 2 × 56 = 112 g (1) OR • 160 g Fe₂O₃ produces 112 g Fe (using ratio from equation) (1) • Therefore 160 g Fe₂O₃ produces 112 g Fe (1) Award (1) for correct molar mass, (1) for correct method, (1) for correct answer

(d) • Actual mass = 112 × 0.85 = 95.2 g (1) Accept: 95 g Award mark if follows through from incorrect answer to (c)

(e) • Carbon atoms are different size (to iron atoms) (1) • This distorts / disrupts the regular arrangement of iron atoms / prevents layers sliding (1) Accept: carbon atoms do not fit in the structure; carbon atoms get between iron atoms

[Total: 9 marks]

Question 4

(a) • carbon dioxide (1) • water (1) Both required for each mark; accept CO₂ and H₂O

(b)(i) • 25-37 °C / 30-40 °C (1) Accept: any range within 20-40 °C

(b)(ii) • To prevent oxygen / air entering (1) • Because oxygen / air would kill the yeast / cause oxidation (to ethanoic acid) / yeast respires aerobically and doesn't produce ethanol (1) Accept: anaerobic conditions are needed

(c)(i) • Crude oil / petroleum / fossil fuels (1) Accept: fractional distillation of crude oil Reject: ethane

(c)(ii) • High temperature / 300 °C / 570 K (1) OR • High pressure / 60-70 atmospheres (1) OR • (Phosphoric acid) catalyst (1) Accept: any one correct condition

(d) • Advantage: renewable / sustainable raw materials / uses waste products / cheaper (1) • Disadvantage: slower / lower purity / requires purification / batch process (1) Accept: any valid advantage and disadvantage from table or own knowledge

[Total: 9 marks]

Question 5

(a) • Copper / silver / gold / platinum (1) Accept: any metal below hydrogen in reactivity series

(b)(i) • Fizzing / effervescence / bubbles / gas produced (1) • Magnesium disappears / dissolves / gets smaller / solution heats up (1) Accept: any two valid observations

(b)(ii) • magnesium + sulfuric acid → magnesium sulfate + hydrogen (1) All parts required; accept H₂ for hydrogen

(c)(i) • Copper (1) Accept: Cu Reject: copper oxide

(c)(ii) • Carbon is more reactive than copper (1) • So carbon can displace / remove oxygen from copper oxide (1) Accept: carbon is higher in reactivity series

(d) • Copper / brown solid / red-brown solid deposited on nail (1) • Blue colour of solution fades / becomes paler / iron nail becomes coated (1) Accept: any two valid observations

[Total: 9 marks]

Question 6

(a)(i) • N₂ + 3H₂ ⇌ 2NH₃ (2) Award (2) for completely correct equation with state symbols or reversible arrow Award (1) for correct formulae but incorrect balancing or missing reversible arrow

(a)(ii) • Iron (1) Reject: platinum

(b)(i) • 15-20% (1) Accept: any value between 15 and 20

(b)(ii) • The forward reaction is exothermic (1) • Increasing temperature favours the endothermic / reverse / backward reaction (1) Accept: Le Chatelier's principle stated Accept: equilibrium shifts to the left

[Total: 6 marks]

Section B — Extended Response

Question 7

(a)(i) (4 marks) • Crude oil is heated / vaporised (1) • Vapours rise up the fractionating column / tower (1) • Temperature decreases / gets cooler up the column (1) • Different fractions condense at different temperatures / heights (1) • Smaller molecules / lower boiling point fractions condense higher up (1) • Larger molecules / higher boiling point fractions condense lower down (1) Award maximum 4 marks from the points above Accept: separation occurs because different boiling points

(a)(ii) (2 marks) • As the number of carbon atoms increases, boiling point increases (1) • Because intermolecular forces / van der Waals forces are stronger / there are more intermolecular forces (1) Accept: larger molecules have stronger intermolecular forces Reject: stronger bonds between molecules (bonds only exist within molecules)

(a)(iii) (4 marks) • Cracking breaks large molecules into smaller molecules (1) • Smaller molecules are more useful as fuels / easier to ignite / burn better (1) • Cracking also produces alkenes which are useful for making polymers / plastics (1) • C₁₀H₂₂ → C₈H₁₈ + C₂H₄ (1) Accept: any valid equation showing cracking of C₁₀H₂₂ into alkane + alkene with correct balancing

(b)(i) (2 marks) • Carbon monoxide is toxic / poisonous (1) • It combines with haemoglobin / prevents oxygen being carried in blood / reduces oxygen transport (1) Reject: carbon monoxide is a pollutant (insufficient)

(b)(ii) (4 marks - levels of response)

Level 2 (3-4 marks): Clear description of how catalytic converter works AND evaluation of effectiveness with reasoning

• Catalytic converter converts carbon monoxide to carbon dioxide / oxidises carbon monoxide (1) • Uses platinum / palladium / rhodium catalyst (1) • Equation: 2CO + O₂ → 2CO₂ (1) • Very effective at reducing emissions when engine is warm (1) • Less effective when engine is cold / when catalyst not at working temperature (1) • Expensive to manufacture / can be stolen / needs to be replaced (1) • Only works with unleaded petrol (1)

Level 1 (1-2 marks): Basic description OR limited evaluation without clear reasoning

Award maximum 4 marks selecting best points from above

[Total: 16 marks]

Question 8

(a)(i) (2 marks) • Water sample A (1) • Because it needs the least soap to form a lather / only 2 cm³ (1) Accept: volume needed doesn't change after boiling

(a)(ii) (2 marks) • Water sample B (1) • Because volume of soap needed decreases after boiling / hardness is removed by boiling (1) Accept: much less soap needed after boiling (18 to 3)

(a)(iii) (1 mark) • Permanent(ly) hard water (1)

(b)(i) (3 marks) • Rainwater contains dissolved carbon dioxide (1) • Carbon dioxide reacts with water to form carbonic acid / an acid (1) • Carbonic acid reacts with calcium carbonate / limestone to form calcium hydrogencarbonate (1) Equation: calcium carbonate + carbonic acid → calcium hydrogencarbonate OR: CaCO₃ + H₂O + CO₂ → Ca(HCO₃)₂ Award (1) for correct word equation if not already awarded

(b)(ii) (3 marks) • Calcium hydrogencarbonate decomposes (on heating) (1) • To form calcium carbonate / limescale / precipitate (1) • Calcium hydrogencarbonate → calcium carbonate + water + carbon dioxide (1) Accept: symbol equation: Ca(HCO₃)₂ → CaCO₃ + H₂O + CO₂

(c)(i) (3 marks) • Sodium carbonate reacts with calcium ions / magnesium ions (1) • To form an insoluble precipitate / calcium carbonate / removes ions from solution (1) • Ionic equation: Ca²⁺ + CO₃²⁻ → CaCO₃ (1) Accept: word equation instead of ionic equation for third mark

(c)(ii) (4 marks - levels of response)

Level 2 (3-4 marks): Clear evaluation with advantages AND disadvantages of both types with developed reasoning

Advantages of hard water: • Contains calcium (ions) which is good for teeth and bones / prevents heart disease (1) • Tastes better / preferred taste (1) • Reduces lead pipes dissolving / protects lead pipes (1)

Disadvantages of hard water: • Forms scum with soap / wastes soap (1) • Forms limescale in kettles and pipes / blocks pipes (1) • Increases heating costs / reduces efficiency of heating systems / kettles take longer to boil (1)

Advantages of soft water: • Lathers easily with soap / less soap needed (1) • No limescale formed (1) • More economical for washing / heating (1)

Award maximum 4 marks showing balanced evaluation

Level 1 (1-2 marks): Limited evaluation showing advantages OR disadvantages without clear comparison

[Total: 18 marks]

Sample Answers with Examiner Commentary

Question 7(b)(ii) — Sample Answers

Grade A (high distinction) answer*

Catalytic converters work by converting harmful carbon monoxide gas into less harmful carbon dioxide. They contain a catalyst made of platinum, palladium or rhodium. The reaction is 2CO + O₂ → 2CO₃. Catalytic converters are very effective at reducing carbon monoxide emissions when the engine is at normal operating temperature, typically reducing emissions by over 90%. However, they have some limitations. They are less effective when the engine is cold because the catalyst needs to be at a high temperature to work properly. This means that short journeys where the engine doesn't fully warm up produce more pollution. Also, catalytic converters are expensive to manufacture and can be stolen because they contain valuable metals. Despite these drawbacks, they have significantly reduced air pollution from vehicles since they became mandatory.

Mark: 4/4

Examiner commentary: This answer achieves full marks in Level 2. It provides a clear description of the catalytic converter's function with the correct equation, identifies specific catalysts, and gives a balanced evaluation. The student explains both effectiveness (with quantification) and limitations with clear reasoning. The answer demonstrates excellent understanding of both the chemistry and real-world application.

Grade C (pass) answer

Catalytic converters change carbon monoxide into carbon dioxide using a catalyst. The equation is CO + O₂ → CO₂. They are quite effective at reducing pollution from cars. However, they don't work when the engine is cold. They are also expensive. Overall they help reduce carbon monoxide in the air which is good for the environment.

Mark: 2/4

Examiner commentary: This answer achieves lower Level 2 marks. The student correctly identifies the conversion of CO to CO₂ and mentions a catalyst, though the equation is not balanced (no credit lost as this is not specifically required). The evaluation includes one valid point about cold engines but lacks development. The comment about expense is mentioned but not explained. The answer would benefit from more specific detail about how effectiveness varies and why catalytic converters are considered successful despite limitations.

Grade E (near miss) answer

Catalytic converters are fitted to car exhausts to clean the air. They remove carbon monoxide from the exhaust fumes by filtering it out. They work quite well but sometimes they get blocked up and need replacing which costs money. The catalyst speeds up the reaction to remove the carbon monoxide.

Mark: 1/4

Examiner commentary: This answer achieves only Level 1. A common misconception is shown: the student thinks catalytic converters work by filtering/removing CO rather than converting it to CO₂. While the student mentions the catalyst speeds up a reaction (credit given), they don't identify what that reaction is. The reference to "blocking up" is not accurate for catalytic converters. To improve, the student needs to understand that chemical conversion occurs (not physical filtration) and should state what CO is converted into.

Question 8(c)(ii) — Sample Answers

Grade A (high distinction) answer*

There are both advantages and disadvantages to using hard water. The main advantage is that hard water contains dissolved calcium ions which are beneficial for health. Calcium helps develop strong bones and teeth, and some studies suggest it may reduce the risk of heart disease. Hard water also tastes better than soft water according to many people. Additionally, hard water can protect against lead poisoning by coating the inside of old lead pipes with a layer of calcium carbonate, preventing the lead from dissolving.

However, hard water has significant disadvantages. It reacts with soap to form scum, which means more soap is wasted and it's harder to create a lather. This makes washing less efficient and more expensive. Hard water also forms limescale when heated, which is calcium carbonate that deposits in kettles, washing machines and heating systems. This limescale reduces the efficiency of heating elements, increases energy costs, and can eventually block pipes completely, leading to expensive repairs.

In contrast, soft water lathers easily with soap and doesn't form limescale, making it more economical for households in the long term despite the lack of health benefits from calcium.

Mark: 4/4

Examiner commentary: This is an excellent Level 2 response that demonstrates comprehensive evaluation. The student presents multiple advantages (health benefits, taste, lead pipe protection) and disadvantages (scum formation, limescale, economic costs) with clear explanations. The comparison with soft water shows good understanding. The answer is well-structured and uses precise terminology throughout.

Grade C (pass) answer

Hard water has some advantages. It contains calcium which is good for your bones and teeth. It also tastes nicer than soft water.

The disadvantages are that hard water forms scum with soap so you need to use more soap. It also causes limescale in kettles which means kettles take longer to boil water. The limescale can damage pipes and heating systems.

Soft water doesn't have these problems because it doesn't contain the calcium and magnesium ions, so it's better for washing and heating.

Mark: 3/4

Examiner commentary: This is a solid Level 2 response. The student identifies key advantages (health benefits, taste) and disadvantages (scum, limescale, heating issues) with reasonable explanations. However, the evaluation could be more developed - for example, explaining why limescale affects heating efficiency or quantifying the economic impact. The comparison with soft water is present but brief. More detail would secure the fourth mark.

Grade E (near miss) answer

Hard water is bad because it doesn't work properly with soap and makes scum in the bath. It also makes kettles go white inside with scale which looks dirty. You have to use more soap with hard water which costs more money.

Soft water is better because it doesn't do this. However hard water is healthier because it has minerals in it which are good for you.

Mark: 2/4

Examiner commentary: This answer achieves Level 1. The student identifies some relevant points (scum formation, limescale, soap usage, health) but the evaluation is undeveloped and imbalanced. A key weakness is the lack of scientific terminology - "looks dirty" and "doesn't work properly" are vague. The student doesn't explain why hard water causes these problems (calcium/magnesium ions reacting) or develop the health benefits (specifically calcium for bones/teeth). The dismissive tone ("hard water is bad") suggests limited understanding of why hard water continues to be supplied. To improve, the student should provide more balanced evaluation with scientific explanations.