Mark Scheme

Section A — Structured Questions (48 marks)

Question 1

(a) CuO + H₂SO₄ → CuSO₄ + H₂O [2]

• Correct equation with all formulae correct [1]
• Balanced [1]
• Accept state symbols but not required

(b) [4] Any four from:

• Add copper(II) oxide to the acid a little at a time / until in excess / until no more dissolves [1]
• Stir / warm to speed up reaction [1]
• Filter to remove excess copper(II) oxide [1]
• Heat/warm the filtrate to evaporate some water / to concentrate the solution [1]
• Leave to crystallise / allow to cool [1]
• Dry the crystals using filter paper / pat dry with absorbent paper / leave in a warm place [1]
• Do NOT accept: dry in an oven / heat strongly (this would decompose the crystals)
• Maximum 4 marks

(c) [3]

• Mr of CuSO₄·5H₂O = 64 + 32 + 64 + 90 = 250 [1]
• Mr of CuSO₄ = 64 + 32 + 64 = 160 [1]
• Mass = (160/250) × 14.8 = 9.47 g / 9.5 g [1]
• Accept answers in range 9.4–9.5 g
• Award [2] for correct answer without working

(d) [2] Any two from:

• Blue crystals / solid turn(s) white [1]
• Steam / water (vapour) given off / condensation seen [1]
• Solid changes texture / becomes powdery [1]
• Reject: "smoke" or "gas"

Question 2

(a) Air / atmosphere [1]

• Accept: fractional distillation of (liquid) air
• Do NOT accept: nitrogen gas alone without source

(b) [3]

• Lower temperature would give a higher yield / move equilibrium to the right / favour the forward (exothermic) reaction [1]
• But the rate of reaction would be (too) slow (at lower temperature) [1]
• 450°C is a compromise between rate and yield / gives a reasonable rate and an acceptable yield [1]
• Accept: reference to economic considerations

(c)(i) Temperature B [1]

(c)(ii) [2]

• The forward reaction is exothermic / ΔH is negative [1]
• Higher temperature favours the endothermic / backward reaction (so yield decreases) / lower temperature favours the exothermic / forward reaction (so yield increases) [1]
• Accept: description of Le Chatelier's principle

(d) [2] Any two from:

• High pressure is expensive / costly to maintain [1]
• Requires stronger / thicker / more expensive equipment / vessels [1]
• High energy costs [1]
• Safety considerations / risk of explosion [1]
• Maximum 2 marks

Question 3

(a)(i) C₆H₁₂O₆ → 2C₂H₅OH + 2CO₂ [2]

• Correct equation [1]
• Balanced [1]

(a)(ii) [2] Any two from:

• Temperature: 25–40°C / warm (not too hot) [1]
• (In the presence of) yeast / enzymes [1]
• Absence of air / anaerobic conditions / exclude oxygen [1]
• Aqueous solution [1]

(a)(iii) Phosphoric acid / H₃PO₄ [1]

• Accept: concentrated phosphoric acid / acid catalyst
• Do NOT accept: sulfuric acid

(b)(i) [2]

• A series of compounds with the same general formula / same functional group [1]
• (Which) differ by CH₂ / have similar chemical properties / show a gradation in physical properties [1]

(b)(ii) [1]

    H   H   H
    |   |   |
H - C - C - C - O - H
    |   |   |
    H   H   H

• All bonds shown correctly [1]
• Accept structural formula: CH₃CH₂CH₂OH

(c)(i) CH₃COOH / C₂H₄O₂ [1]

• Accept either molecular formula

(c)(ii) Carboxyl (group) / carboxylic acid (group) / –COOH [1]

• Do NOT accept: just "acid"

Question 4

(a) The number of protons (in the nucleus of an atom) [1]

• Accept: proton number
• Do NOT accept: just "number of protons" without reference to nucleus / atom

(b)(i) Group 1 / Group I [1]

(b)(ii) [2]

• Group (number) is the same as the number of electrons in the outer shell / outermost shell [1]
• Sodium has 1 electron in its outer shell (so it is in Group 1) [1]

(c)(i) 2Na(s) + 2H₂O(l) → 2NaOH(aq) + H₂(g) [3]

• Correct equation [1]
• Balanced [1]
• All state symbols correct [1]

(c)(ii) [2] Any two from:

• Sodium melts / forms a ball [1]
• Sodium moves / fizzes / floats (on the water) [1]
• Gas / bubbles / effervescence / hydrogen produced [1]
• Heat is given out / exothermic [1]
• Sodium disappears / dissolves [1]
• (Colourless) solution formed [1]
• If universal indicator added: solution turns blue / purple [1]
• Maximum 2 marks

(d) [3]

• Caesium is more reactive (than sodium) [1]
• Caesium has more shells / electron shells / energy levels (than sodium) / outer electron is further from the nucleus [1]
• Outer electron is more easily lost / weaker attraction between nucleus and outer electron / more shielding / greater atomic radius [1]
• Accept reverse argument for sodium being less reactive

Question 5

(a) A mixture of a metal with one or more other elements [1]

• Accept: mixture of (two or more) metals / mixture of metals or non-metals
• Must mention "mixture"

(b)(i) Carbon / carbon monoxide / coke / C / CO [1]

• Accept either

(b)(ii) Fe₂O₃ + 3CO → 2Fe + 3CO₂ [2]

• Correct formulae [1]
• Balanced [1]

(c) [3]

• Steel contains atoms of different sizes / carbon atoms are different size to iron atoms [1]
• (Different sized atoms) distort the layers / disrupt the regular arrangement [1]
• (So) layers cannot slide over each other (as easily) [1]
• Accept: reference to carbon atoms fitting between iron atoms / in gaps

(d) Chromium / other elements / alloying elements prevent / reduce corrosion / oxidation / reaction with oxygen and water [1]

• Accept: forms a protective layer / oxide layer

(e) [2]

• Moles = concentration × volume (in dm³) [1]
• = 0.0200 × (20.0/1000) = 0.000400 mol / 4.00 × 10⁻⁴ mol [1]
• Accept: 0.0004 mol
• Award [2] for correct answer without working

Question 6

(a) [3]

• Crude oil is heated / vaporised [1]
• Vapours rise up the fractionating column / tower [1]
• Different fractions condense at different temperatures / heights (because they have different boiling points) [1]
• Accept: fractions with lower boiling points condense higher up / smaller molecules condense at top

(b)(i) Thermal decomposition / decomposition [1]

• Do NOT accept: just "cracking"

(b)(ii) [2]

• C₅H₁₂ [2]
• Award [1] for correct method shown: working out from atom conservation
• Carbon: 10 = 3 + 2 + x, so x = 5
• Hydrogen: 22 = 6 + 4 + y, so y = 12

(c)(i) Bromine water changes from orange / brown / yellow to colourless [1]

• Accept: decolourises / loses colour / goes clear
• Must mention both initial and final colour for full mark
• Accept: fades

(c)(ii) [2]

• Alkenes contain a C=C double bond / carbon–carbon double bond [1]
• Double bond / C=C is reactive / can undergo addition reactions / alkanes only have single bonds which are less reactive [1]
• Do NOT accept: just "alkenes are reactive" without reference to double bond

(d) [2]

    H   H   H   H
    |   |   |   |
- C - C - C - C -
    |   |   |   |
    H   H   H   H

OR

  H   H   H   H
  |   |   |   |
  C - C - C - C
  |   |   |   |
  H   H   H   H
     n

• Two repeat units shown [1]
• Continuation bonds shown correctly [1]
• Accept displayed formula or condensed structural formula showing continuation

Section B — Extended Response (32 marks)

Question 7

(a) [4] Level 2 (3–4 marks):

• Identifies that Experiment 2 is faster [1]
• Explains that powder has a larger surface area (than large chips) [1]
• Links to collision theory: more particles exposed / more frequent collisions [1]
• States that more collisions lead to a faster reaction [1]

Level 1 (1–2 marks):

• States Experiment 2 is faster [1]
• May mention surface area or collisions but explanation incomplete [1]

(b) [3]

• Yes, same volume of CO₂ produced [1]
• Same mass of calcium carbonate / CaCO₃ / marble chips used [1]
• Calcium carbonate is the limiting reactant / all the calcium carbonate reacts / same amount of product formed [1]
• Accept: same number of moles of CaCO₃

(c)(i) [3] Any three from:

• Concentration of acid decreases (as reaction proceeds) [1]
• Number of (successful) collisions per unit time decreases [1]
• Rate of reaction decreases [1]
• Particles of calcium carbonate / reactant are used up / less calcium carbonate available [1]
• Surface area of marble chips decreases [1]
• Maximum 3 marks

(c)(ii) [2]

• Draw a tangent to the curve at 60 seconds [1]
• Calculate the gradient / slope of the tangent / measure change in y ÷ change in x [1]
• Accept: appropriate description of how to calculate gradient

(d) [2]

• Curve starts at same point (origin) [1]
• Curve is steeper initially and levels off at same final volume but reaches it sooner / curve reaches plateau earlier [1]
• Must show curve levelling off at approximately same height as Experiment 1

(e) [4] Level 2 (3–4 marks): Comprehensive explanation including:

• Particles have more (kinetic) energy [1]
• Particles move faster / collide more frequently [1]
• More particles have energy greater than / equal to the activation energy [1]
• More collisions are successful / lead to reaction [1]

Level 1 (1–2 marks): Basic explanation:

• Mentions particles having more energy OR moving faster [1]
• May mention increased collisions but no link to activation energy [1]

Question 8

(a)(i) Below 7 / pH less than 7 / acidic [1]

(a)(ii) [3]

• Carbon dioxide dissolves in water / reacts with water [1]
• Forms carbonic acid / H₂CO₃ [1]
• CO₂ + H₂O → H₂CO₃ [1]
• Accept: H₂CO₃ ⇌ H⁺ + HCO₃⁻ / dissociates to form H⁺ ions
• Accept: weak acid formed / slightly acidic

(b)(i) [2] Any one from (with appropriate detail):

• Scale / limescale forms (in kettles / pipes / heating elements) [1] which reduces efficiency / blocks pipes / increases energy costs / needs to be removed [1]
• More soap / detergent needed / soap does not lather easily [1] which increases cost / is wasteful [1]
• Maximum 2 marks for one problem well explained

(b)(ii) [3]

• (Temporary hardness is caused by) calcium hydrogencarbonate / Ca(HCO₃)₂ (in solution) [1]
• Heating causes it to decompose [1]
• Ca(HCO₃)₂ → CaCO₃ + H₂O + CO₂ [1]
• Accept: calcium carbonate / CaCO₃ forms / precipitates (which can be removed)
• Accept correct equation with heat shown

(b)(iii) [1] Any one from:

• Good for teeth / bones / prevents tooth decay [1]
• Provides calcium (which is needed for health) [1]
• Can help prevent heart disease [1]
• Do NOT accept: general "good for health" without specific benefit

(c)(i) [2]

• Chlorination / addition of chlorine / sterilisation / disinfection [1]
• To kill bacteria / microorganisms / pathogens / germs (to make water safe to drink) [1]
• Accept: other named sterilising agent (e.g., ozone, UV light)

(c)(ii) [2] Advantage (any one from):

• Reduces tooth decay / strengthens teeth / good for dental health [1]

Disadvantage (any one from):

• Can cause discolouration of teeth / fluorosis (at high concentrations) [1]
• Some people object on ethical grounds / believe it is mass medication / should be a personal choice [1]
• Cost of adding fluoride [1]

Sample Answers with Examiner Commentary

Question 7 — Sample Answers

Grade A (high distinction) answer*

(a) Experiment 2 will have a faster initial rate of reaction. This is because the marble chips are in powder form, which has a much larger surface area than large chips. According to collision theory, when the surface area is increased, more calcium carbonate particles are exposed to the acid. This means there are more frequent collisions between the acid particles and the calcium carbonate particles per second. Because there are more collisions per unit time, the rate of reaction is faster.

(b) Yes, experiments 1 and 2 will produce the same total volume of carbon dioxide. This is because both experiments use the same mass (5.0 g) of calcium carbonate, which means the same number of moles of calcium carbonate. Since the calcium carbonate is the limiting reactant, all of it will react in both experiments to produce the same number of moles of carbon dioxide, and therefore the same volume of carbon dioxide gas.

(c)(i) The gradient of the curve decreases as the reaction proceeds because the concentration of hydrochloric acid decreases as it is used up in the reaction. As the concentration decreases, the number of acid particles per unit volume decreases, so there are fewer collisions between reactant particles per second. This means the rate of reaction decreases. Additionally, the surface area of the marble chips decreases as they are used up, further reducing the number of collisions.

(c)(ii) To determine the rate at 60 seconds, I would draw a tangent to the curve at exactly 60 seconds. Then I would calculate the gradient of this tangent by measuring the change in volume of CO₂ (vertical axis) divided by the change in time (horizontal axis). The gradient gives the rate of reaction at that specific time in cm³/s.

(d) [The sketch would show a curve starting at the origin, rising more steeply than the original curve, but levelling off at the same final volume of approximately 100 cm³, reaching the plateau sooner than Experiment 1]

(e) Increasing the temperature increases the rate of reaction for two main reasons. First, when the temperature increases, the particles have more kinetic energy, so they move faster. This means they collide more frequently, leading to more collisions per second. Second, and more importantly, at higher temperatures more particles have energy equal to or greater than the activation energy needed for the reaction to occur. This means that a greater proportion of the collisions are successful and result in a reaction. Both of these factors combined lead to an increased rate of reaction at higher temperatures.

Mark: 18/18

Examiner commentary: This is an exemplary answer demonstrating complete understanding of rates of reaction and collision theory. The student consistently uses precise scientific terminology, makes clear links between concepts, and provides comprehensive explanations. In part (e), the candidate distinguishes between collision frequency and collision energy, showing sophisticated understanding of the relative importance of these factors—a hallmark of A* responses.

Grade C (pass) answer

(a) Experiment 2 will be faster because the powder has more surface area than the large chips. This means more particles can react at once. There will be more collisions so the reaction is faster.

(b) Yes they will produce the same amount of CO₂. Both experiments use 5.0g of marble chips so the same amount will react.

(c)(i) The gradient decreases because the acid is being used up. As the reaction goes on there is less acid left so less collisions happen. This makes the reaction slower.

(c)(ii) Draw a tangent at 60 seconds and work out the gradient.

(d) [The sketch would show a steeper curve but might not level off at exactly the same height, or the plateau might not be clearly earlier]

(e) When you increase the temperature the particles have more energy and move faster. This means they collide more often. The collisions are also harder so more of them cause a reaction. This makes the rate increase.

Mark: 10/18

Examiner commentary: This answer demonstrates sound basic understanding but lacks the depth and precision required for higher marks. In part (a), the student correctly identifies the key factor (surface area) and makes the connection to collisions, but the explanation is less detailed (awarded 3/4 marks). Part (b) is correct but would benefit from mentioning limiting reactant or moles. Part (c)(i) gains 2/3 marks—the student understands concentration decreases but doesn't explain the mechanism as clearly. Part (e) gains 2/4 marks as the student mentions energy and collision frequency but doesn't explicitly discuss activation energy, which is a key concept at this level.

Grade E (near miss) answer

(a) Experiment 2 is faster because powder is smaller so it reacts quicker. Small pieces react faster than big pieces.

(b) No, experiment 2 will make more gas because it reacts faster so more CO₂ is produced.

(c)(i) The line goes flat because the reaction has finished. All the reactants are used up so no more gas is made.

(c)(ii) Measure the amount of gas at 60 seconds.

(d) [The sketch might show a steeper curve that continues rising to a higher final volume, showing a common misconception]

(e) Higher temperature makes particles move faster so they bump into each other more. More bumping means faster reaction.

Mark: 5/18

Examiner commentary: This response shows basic awareness of rates of reaction but contains significant misconceptions and lacks scientific precision. In part (a), the student correctly identifies that Experiment 2 is faster but doesn't use the term "surface area" or explain using collision theory (awarded 1/4). A critical error appears in part (b)—confusing rate with total yield, a common IGCSE misconception (0 marks). Part (c)(i) confuses the gradient decreasing with the reaction stopping (1/3 marks). The student doesn't understand tangents in part (c)(ii). Part (e) is too vague and doesn't mention activation energy. To improve, this student needs to learn key terminology (surface area, activation energy, limiting reactant) and understand that rate and total yield are different concepts.

Question 8 — Sample Answers

Grade A (high distinction) answer*

(a)(i) Below 7 / acidic

(a)(ii) When carbon dioxide dissolves in water it reacts with the water to form carbonic acid, H₂CO₃. The equation for this is: CO₂ + H₂O → H₂CO₃. Carbonic acid is a weak acid that partially dissociates to release hydrogen ions (H⁺) into the solution, which makes the pH less than 7.

(b)(i) One problem caused by hard water is that scale or limescale forms inside kettles, hot water pipes, and on heating elements. This scale acts as an insulator and reduces the efficiency of heating, meaning more energy is needed to heat water, which increases costs. The scale can also eventually block pipes completely.

(b)(ii) Temporary hardness is caused by dissolved calcium hydrogencarbonate, Ca(HCO₃)₂. When the water is boiled, the calcium hydrogencarbonate thermally decomposes into insoluble calcium carbonate, water, and carbon dioxide: Ca(HCO₃)₂ → CaCO₃ + H₂O + CO₂. The calcium carbonate precipitate can then be removed by filtration, which removes the hardness.

(b)(iii) Hard water is beneficial because it contains calcium ions which are good for the development of strong bones and teeth.

(c)(i) Stage X is chlorination, where chlorine gas or compounds are added to the water. This is necessary to kill harmful bacteria, viruses, and other microorganisms that could cause disease, making the water safe to drink.

(c)(ii) One advantage of adding fluoride to water supplies is that it significantly reduces tooth decay and strengthens tooth enamel, especially in children. One disadvantage is that some people believe it is unethical to add fluoride as it removes personal choice about medication, and at high concentrations it can cause fluorosis, which is discolouration of teeth.

Mark: 14/14

Examiner commentary: An outstanding response showing comprehensive knowledge of water chemistry. The student provides precise chemical detail throughout, including correct formulae and equations. In part (b)(i), the answer doesn't just state a problem but explains its consequences, demonstrating sophisticated understanding. Part (b)(ii) shows the ability to link chemical concepts (thermal decomposition) to practical applications. The ethical dimension in part (c)(ii) shows awareness beyond pure chemistry, which is valued in extended response questions.

Grade C (pass) answer

(a)(i) Below 7

(a)(ii) Carbon dioxide dissolves in water and makes an acid called carbonic acid. This makes the water acidic so the pH goes down below 7.

(b)(i) Hard water causes scale to form in kettles. This is a white solid that builds up and makes kettles less efficient.

(b)(ii) Boiling makes the calcium hydrogencarbonate break down into calcium carbonate which is insoluble. This forms a precipitate which can be filtered out. The equation is: Ca(HCO₃)₂ → CaCO₃ + H₂O + CO₂

(b)(iii) It is good for bones and teeth.

(c)(i) Chlorine is added to kill bacteria and make the water safe.

(c)(ii) Advantage: reduces tooth decay. Disadvantage: some people don't want it added to their water.

Mark: 8/14

Examiner commentary: A solid mid-level response that covers the key points but often lacks depth. In part (a)(ii), the student correctly identifies carbonic acid but doesn't provide the equation (awarded 2/3). Part (b)(i) describes scale formation but could be more detailed about consequences (1/2 marks awarded). Part (b)(ii) is good and gains full marks. Part (b)(iii) is correct but brief. Parts (c)(i) and (c)(ii) are acceptable but could be expanded—the disadvantage in (c)(ii) lacks specific detail. To reach higher grades, this student should develop answers more fully and include chemical equations where relevant.

Grade E (near miss) answer

(a)(i) Less than 7

(a)(ii) CO₂ makes the water acidic because it is an acidic gas.

(b)(i) It makes scum with soap.

(b)(ii) When you boil hard water the calcium comes out and forms solid bits at the bottom of the kettle. This removes the hardness.

(b)(iii) It tastes better.

(c)(i) Chlorine is added. It kills germs.

(c)(ii) Advantage: kills bacteria. Disadvantage: fluoride is poisonous.

Mark: 4/14

Examiner commentary: This response demonstrates fragmented understanding with several misconceptions. Part (a)(ii) shows a common error—describing CO₂ itself as acidic rather than explaining acid formation (1/3 marks). In part (b)(i), the student confuses two different problems of hard water; scum formation is correct but scale/limescale would be a more relevant answer for the marks available. Part (b)(ii) shows vague understanding but lacks the key terms "calcium hydrogencarbonate" and "thermal decomposition," and no equation (1/3 marks). Part (b)(iii) is incorrect—taste is not a recognized health benefit. Part (c)(ii) confuses chlorine with fluoride and incorrectly states fluoride is poisonous. This student needs to review water chemistry systematically, learn correct terminology, and distinguish between different concepts that are easily confused.