Mark scheme

Use this to mark your own attempt. Accept any correct alternative that answers the question as set. Marks are for the ideas, not the exact words. Unless stated, ecf (error carried forward) applies in calculations. Chermistry practice paper – not an official examination paper.

Textbook page references are to Discover Chemistry GCE 'O' Level Science and Normal (A) Level Science Textbook, 3rd Edition (Marshall Cavendish Education, 2023).

Question Number	Key
1	C
2	B
3	D
4	A
5	C

Question Number	Key
6	C
7	A
8	D
9	D
10	B

Question Number	Key
11	C
12	A
13	B
14	A
15	D

Question Number	Key
16	A
17	B
18	C
19	B
20	D

Comments on specific questions

Question 1

C: the delivery tube reaches the top of an upside-down jar, so the gas collects at the top and pushes the air out of the bottom: only a gas that is less dense than air (hydrogen, M_r 2) can be collected this way. A, B, D: carbon dioxide (44), chlorine (71) and sulfur dioxide (64) are denser than air (about 29), so they would sink and leave the jar through the open mouth; they are collected by downward delivery (jar upright).

Textbook: Discover Chemistry GCE 'O' Level Science and Normal (A) Level Science Textbook, 3rd Edition (Marshall Cavendish Education, 2023), pp. 6–7

Question 2

B: on melting the particles gain energy, break out of their fixed regular positions and move around in a random arrangement. A: the arrangement is right but energy is taken in, not lost (it describes the reverse energy change). C, D: the arrangement change is that of freezing; C is freezing in full, D mixes the freezing arrangement with the melting energy change.

Textbook: Discover Chemistry GCE 'O' Level Science and Normal (A) Level Science Textbook, 3rd Edition (Marshall Cavendish Education, 2023), pp. 24–34

Question 3

D: protons = proton number = 15; neutrons = 31 − 15 = 16; the ion has gained 3 electrons, so electrons = 15 + 3 = 18. A: treats 3− as three electrons lost (15 − 3 = 12). B: uses the nucleon number (31) as the number of neutrons. C: the numbers of protons and neutrons are swapped (16 and 15).

Textbook: Discover Chemistry GCE 'O' Level Science and Normal (A) Level Science Textbook, 3rd Edition (Marshall Cavendish Education, 2023), pp. 40–42

Question 4

A: each X atom has lost 1 electron to form X⁺, so X has 1 outer electron (Group 1); the Y atom has gained 2 electrons to form Y²⁻ with a full shell of 8, so it had 6 outer electrons (Group 16). The formula is X₂Y. B: Y is taken to be Group 17, a halogen (one electron gained, as in a Group 17 ion of 1−). C: X is given the Group number 2 because there are 2 X ions in the diagram (the number of ions is confused with the charge). D: both mistakes together.

Textbook: Discover Chemistry GCE 'O' Level Science and Normal (A) Level Science Textbook, 3rd Edition (Marshall Cavendish Education, 2023), pp. 58–59

Question 5

C: R has a low melting point (weak forces between molecules) and has no charged particles free to move in either state, so it does not conduct when solid or molten. A: P has a high melting point and conducts only when molten: an ionic compound (ions free to move only when molten). B: Q conducts when solid: a metal (metals conduct when solid and when molten). D: S is a metal with a high melting point.

Textbook: Discover Chemistry GCE 'O' Level Science and Normal (A) Level Science Textbook, 3rd Edition (Marshall Cavendish Education, 2023), pp. 78–80

Question 6

C: 0.10 mol of molecules = 0.10 × 6.02 × 10²³ = 6.02 × 10²² molecules (Avogadro constant from the Periodic Table page); each molecule has 5 atoms (1 C + 4 H), so 5 × 6.02 × 10²² = 3.01 × 10²³ atoms. A: counts molecules, not atoms. B: counts only the hydrogen atoms (4 per molecule: 4 × 6.02 × 10²² = 2.41 × 10²³). D: uses 1.0 mol instead of 0.10 mol (a factor of 10 too large).

Textbook: Discover Chemistry GCE 'O' Level Science and Normal (A) Level Science Textbook, 3rd Edition (Marshall Cavendish Education, 2023), pp. 108–109

Question 7

A: n(Mg) = 0.48 ÷ 24 = 0.020 mol; n(HCl) = 0.020 × 0.50 = 0.010 mol. 0.020 mol Mg would need 0.040 mol HCl, so the acid is limiting. n(H₂) = 0.010 ÷ 2 = 0.0050 mol; volume = 0.0050 × 24 000 = 120 cm³. B: takes the ratio HCl : H₂ as 1 : 1 (0.010 mol). C: uses the magnesium (0.020 mol) and ignores that it is in excess. D: uses magnesium and multiplies by the 2 from the coefficient of HCl (0.040 mol).

Textbook: Discover Chemistry GCE 'O' Level Science and Normal (A) Level Science Textbook, 3rd Edition (Marshall Cavendish Education, 2023), pp. 114–119

Question 8

D: diluting an acid lowers the concentration of H⁺ ions, so the solution becomes less acidic and the pH rises towards 7. A: the concentration change is right but the pH is said to fall (a falling pH would mean the solution is becoming more acidic). B: both changes are the wrong way round (it describes adding more acid). C: the pH change is right but the H⁺ concentration is said to rise (a low pH means a high H⁺ concentration, so a rising pH means a falling concentration).

Textbook: Discover Chemistry GCE 'O' Level Science and Normal (A) Level Science Textbook, 3rd Edition (Marshall Cavendish Education, 2023), pp. 129–131

Question 9

D: zinc oxide is amphoteric: it reacts with both acids and alkalis. A: calcium oxide is a basic oxide (a metal oxide) and reacts with the acid only. B: carbon monoxide is a neutral oxide and reacts with neither. C: sulfur dioxide is an acidic oxide (a non-metal oxide) and reacts with the alkali only.

Textbook: Discover Chemistry GCE 'O' Level Science and Normal (A) Level Science Textbook, 3rd Edition (Marshall Cavendish Education, 2023), pp. 134–136

Question 10

B: warming with aqueous sodium hydroxide gives ammonia (turns damp red litmus paper blue), so the cation is NH₄⁺; a white precipitate with acidified barium nitrate shows sulfate. A: ammonium is right but chloride gives no precipitate in test 2. C, D: calcium ions give a white precipitate with sodium hydroxide but no ammonia gas on warming (C also has the wrong anion).

Textbook: Discover Chemistry GCE 'O' Level Science and Normal (A) Level Science Textbook, 3rd Edition (Marshall Cavendish Education, 2023), pp. 142–144

Question 11

C: acidified manganate(VII) loses its purple colour when it is reduced, so it is the oxidising agent and the iron(II) ions are the reducing agent: they lose electrons and are oxidised (to iron(III)). A: the iron(II) ions are correctly said to be oxidised, but the role is swapped (an oxidising agent is itself reduced). B: both ideas are reversed. D: the role is right, but a reducing agent is itself oxidised, not reduced.

Textbook: Discover Chemistry GCE 'O' Level Science and Normal (A) Level Science Textbook, 3rd Edition (Marshall Cavendish Education, 2023), pp. 156–161

Question 12

A: elements in the same group have the same number of outer shell electrons (1 for Group 1), and this decides how they react. B: sodium has 3 shells and potassium has 4 (the number of shells = the period, not the group). C: sodium has 12 neutrons and potassium 20. D: sodium has 11 protons and potassium 19.

Textbook: Discover Chemistry GCE 'O' Level Science and Normal (A) Level Science Textbook, 3rd Edition (Marshall Cavendish Education, 2023), p. 172

Question 13

B: reactivity increases down Group 1, so potassium reacts most vigorously; the products are hydrogen and the metal hydroxide, an alkaline solution. A: lithium is the least reactive of the three, and the solution is not acidic. C: sodium is more reactive than lithium but less reactive than potassium. D: the metal is right but the solution is alkaline, not acidic.

Textbook: Discover Chemistry GCE 'O' Level Science and Normal (A) Level Science Textbook, 3rd Edition (Marshall Cavendish Education, 2023), p. 175

Question 14

A: Q reacts with cold water, so it is the most reactive; R reacts rapidly with acid and P only slowly, so R is more reactive than P; S shows no reaction with either. B: P and R are swapped (slow and rapid reaction with acid misread). C: ranks R above Q, ignoring that only Q reacts with cold water. D: the order is reversed (least reactive first).

Textbook: Discover Chemistry GCE 'O' Level Science and Normal (A) Level Science Textbook, 3rd Edition (Marshall Cavendish Education, 2023), pp. 186–187

Question 15

D: the temperature of the mixture falls, so energy is taken in from the surroundings: the process is endothermic. A: dissolving is assumed to be exothermic and to give out energy. B: the energy direction is right but the process is called exothermic. C: calls it endothermic but gives the direction of energy transfer of an exothermic change.

Textbook: Discover Chemistry GCE 'O' Level Science and Normal (A) Level Science Textbook, 3rd Edition (Marshall Cavendish Education, 2023), pp. 201–204

Question 16

A: a higher temperature gives a faster rate (steeper curve, plateau reached sooner) because the particles move faster and collide more often; the acid is the limiting reactant, so the final volume of hydrogen is unchanged (480 cm³). B: faster, but a larger final volume: the amount of acid has not changed. C: same final volume but slower: the effect of temperature on the rate is reversed. D: slower and a smaller volume (a lower temperature, and the idea that a slower reaction gives less product).

Textbook: Discover Chemistry GCE 'O' Level Science and Normal (A) Level Science Textbook, 3rd Edition (Marshall Cavendish Education, 2023), pp. 214–221

Question 17

B: the carbon dioxide released when bioethanol burns is offset by the carbon dioxide absorbed by the sugarcane as it grew. A: ethanol contains carbon, so carbon dioxide forms when it burns. C: carbon dioxide forms in complete combustion; incomplete combustion forms carbon monoxide (or carbon) as well. D: ethanol contains carbon, so burning it forms carbon dioxide as well as water.

Textbook: Discover Chemistry GCE 'O' Level Science and Normal (A) Level Science Textbook, 3rd Edition (Marshall Cavendish Education, 2023), p. 233

Question 18

C: the atoms must balance: carbon 12 − 2 − 4 = 6; hydrogen 26 − 4 − 8 = 14, so X is C₆H₁₄, an alkane (C_nH_2n+2). A, B: have too few hydrogen atoms; they would need extra hydrogen as a product (B is an alkene, the idea that cracking always makes alkenes). D: C₆H₁₆ does not exist (an alkane with 6 C has at most 14 H) and would not balance.

Textbook: Discover Chemistry GCE 'O' Level Science and Normal (A) Level Science Textbook, 3rd Edition (Marshall Cavendish Education, 2023), pp. 250–252

Question 19

B: in addition polymerisation the C=C double bond of each monomer opens, so the chain has only single C–C bonds and each carbon keeps its two F atoms. A: the C=C double bonds are kept (an addition polymer has none) and half of the F atoms are missing. C: each carbon has one F and one H, which is not the monomer Z. D: all H atoms: this is the polymer of ethene (poly(ethene)), not of Z.

Textbook: Discover Chemistry GCE 'O' Level Science and Normal (A) Level Science Textbook, 3rd Edition (Marshall Cavendish Education, 2023), pp. 277–278

Question 20

D: sulfur dioxide dissolves in rain water to give acidic solutions: acid rain. A: carbon monoxide forms by incomplete combustion; complete combustion gives carbon dioxide. B: methane is a greenhouse gas; acid rain is caused by sulfur dioxide and oxides of nitrogen. C: lightning is a source of oxides of nitrogen (nitrogen and oxygen of the air react at the high temperature); sulfur dioxide comes from volcanoes and from burning fossil fuels that contain sulfur.

Textbook: Discover Chemistry GCE 'O' Level Science and Normal (A) Level Science Textbook, 3rd Edition (Marshall Cavendish Education, 2023), pp. 293–294

SCIENCE (CHEMISTRY) – Paper 1 Multiple Choice (Chemistry section) – mark scheme