Mark Scheme

Section A — Structured Questions

1. (a) Protons = 12; neutrons = 24 − 12 = 12; electrons = 12 (1 each). [3] (b) Group 2 because it has 2 electrons in its outer shell (1); Period 3 because it has 3 occupied electron shells (1). [2] (c) (up to 3) The magnesium atom loses 2 electrons (from its outer shell) to form a Mg²⁺ ion; the oxygen atom gains 2 electrons to form an O²⁻ ion; the electrons are transferred from magnesium to oxygen, forming oppositely charged ions. [3]

2. (a) (up to 2) There are strong electrostatic forces of attraction between the oppositely charged ions; a large amount of energy is needed to overcome these forces, so the melting point is high. [2] (b) (up to 3) When solid, the ions are fixed in place / cannot move, so it cannot conduct; when molten or dissolved, the ions are free to move and carry charge, so it conducts. [3] (c) (up to 4) In graphite, each carbon atom is bonded to three others, leaving one delocalised electron per atom that is free to move and carry charge, so it conducts; in diamond, each carbon is bonded to four others with no free electrons, so it does not conduct. [4]

3. (a) Strong acid pH 0–2 (accept 1–2) (1); indicator — e.g. litmus (turns red) / methyl orange / universal indicator (1). [2] (b) Products: magnesium chloride + water + carbon dioxide (1 each for correct products in the right places); gas = carbon dioxide (implicit). [3] (c) (up to 2) Bubble the gas through limewater; if it is carbon dioxide, the limewater turns cloudy/milky. [2] (d) (up to 3) Add excess magnesium carbonate to the acid so all the acid reacts; filter to remove the unreacted excess solid; evaporate some of the water from the filtrate and leave the solution to crystallise, then dry the crystals. [3]

4. (a) Any two: same volume/concentration of acid; same mass of marble; same temperature; same size of chips (for the first run). [2] (b) (up to 3) Powder has a larger surface area (than chips of the same mass); more acid particles can collide with the marble per second; so the frequency of successful collisions increases and the rate is faster. [3] (c) (up to 3) Increasing temperature gives the particles more kinetic energy, so they move faster and collide more frequently; a greater proportion of collisions have energy above the activation energy, so more collisions are successful and the rate increases. [3] (d) (up to 2) As the reaction proceeds, the reactants are used up, so their concentration/amount falls and collisions become less frequent, slowing the rate; it stops when one reactant (the acid or the marble) is completely used up. [2]

5. (a) Mr(CaCO₃) = 40 + 12 + (3 × 16) = 40 + 12 + 48 = 100 (M1 A1). [2] (b) % Ca = (40 ÷ 100) × 100 = 40% (M1 A1). [2] (c) Moles CaCO₃ = 25 ÷ 100 = 0.25 mol (M1); ratio CaCO₃ : CaO = 1 : 1, so moles CaO = 0.25 (M1); mass CaO = 0.25 × 56 = 14 g (A1). [3]

Section B — Extended Response

6. (a) (up to 4) Aluminium is more reactive than carbon, so carbon cannot displace it from its oxide; it must be extracted by electrolysis. Iron is less reactive than carbon, so carbon can reduce iron oxide (remove the oxygen) in a blast furnace, which is cheaper than electrolysis. Clear link between reactivity and method. [4] (b) (up to 6) At the cathode (negative), aluminium ions gain electrons (reduction: Al³⁺ + 3e⁻ → Al) and molten aluminium forms; at the anode (positive), oxide ions lose electrons (oxidation) forming oxygen; the oxygen reacts with the hot carbon anode to form carbon dioxide, so the anode gradually burns away and must be replaced. Award for cathode process, anode process, and the reason for replacement. [6]

7. (up to 6) Indicative content: the early atmosphere was formed largely by volcanic activity and was rich in carbon dioxide with little oxygen; water vapour condensed to form the oceans; oxygen increased as plants/algae photosynthesised, and carbon dioxide decreased (absorbed by plants and dissolved in oceans/locked in sediments). Human activities — burning fossil fuels and deforestation — are increasing carbon dioxide (and other greenhouse gases), enhancing the greenhouse effect and causing global warming/climate change, with consequences such as rising sea levels and changing weather. Levels: 5–6 = detailed, accurate, both parts covered; 3–4 = several points; 1–2 = basic. [6]

Sample Answers with Examiner Commentary

Question 2(c) — Sample Answers

Grade A response. "Graphite and diamond are both made only of carbon atoms, but they are bonded differently. In graphite, each carbon atom forms covalent bonds to only three other carbon atoms, arranged in layers. This leaves one electron per carbon atom that is not used in bonding, so it becomes delocalised and free to move through the structure. These delocalised electrons can carry charge, which is why graphite conducts electricity. In diamond, each carbon atom is bonded to four other carbon atoms in a rigid three-dimensional structure. All the outer electrons are used in bonding, so there are no free electrons to carry charge, and diamond does not conduct electricity."

Mark: 4/4. Examiner commentary: A complete answer that earns all four marks by making the direct link between structure and property for both forms. The key discriminators are handled precisely: three bonds in graphite leaving one delocalised electron per atom, versus four bonds in diamond using all the outer electrons. Weaker answers often state that graphite "has free electrons" without explaining why (the three-bond arrangement), or describe the structures without connecting them to conduction. This response does both.

Grade C response. "Graphite conducts electricity because it has free electrons that can move. Diamond does not conduct because its atoms are all joined together tightly so nothing can move."

Mark: 2/4. Examiner commentary: The candidate correctly states that graphite has free (delocalised) electrons that allow it to conduct, which earns marks, and knows diamond does not conduct. However, the answer does not explain the difference in bonding that causes this — that graphite's carbons form only three bonds (leaving a spare electron) while diamond's form four (using all the electrons). "Joined together tightly so nothing can move" is imprecise; it is the absence of free electrons, not the rigidity, that prevents conduction. Explaining the number of bonds in each would gain the remaining marks.

Question 5(c) — Sample Answers

Grade A response. "First I need the moles of calcium carbonate. Moles = mass ÷ Mr = 25 ÷ 100 = 0.25 mol. The equation CaCO₃ → CaO + CO₂ shows a 1 : 1 ratio between calcium carbonate and calcium oxide, so the moles of CaO produced are also 0.25 mol. Finally, mass = moles × Mr = 0.25 × 56 = 14 g of calcium oxide."

Mark: 3/3. Examiner commentary: A model calculation. The candidate sets it out in the three standard steps — mass to moles, apply the 1 : 1 ratio from the balanced equation, then moles back to mass — and shows the working at each stage. Because the method is fully visible, all three marks are secure even if an arithmetic slip had occurred. Quoting the ratio explicitly from the equation is exactly what examiners look for.

Grade C response. "25 g of CaCO₃ makes CaO. The Mr of CaO is 56 and CaCO₃ is 100, so 25 × 56 ÷ 100 = 14 g."

Mark: 2/3. Examiner commentary: Interestingly, the candidate reaches the correct answer of 14 g, but by a compressed method that does not clearly show the moles step. The scaling 25 × 56 ÷ 100 happens to work here because the ratio is 1 : 1, and the candidate earns two marks for a correct route and answer. However, the method mark for working in moles (25 ÷ 100 = 0.25 mol, then × 56) is not clearly demonstrated, and this shortcut would fail on a reaction with a ratio other than 1 : 1. Setting the calculation out in moles is safer and gains full marks.