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	A
2	B
3	D
4	A
5	C

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

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

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

Comments on specific questions

Question 1Easy

A: a gas syringe is graduated and the gas never touches water, so it does not matter that hydrogen chloride is very soluble. B: the measuring cylinder is graduated, but the hydrogen chloride would dissolve in the water, so the volume measured would be far too small. C: downward delivery does collect a gas that is denser than air, but a gas jar is not graduated, so the volume still cannot be read. D: a gas jar is not graduated, and upward delivery is for gases that are less dense than air; hydrogen chloride would sink and escape from the open bottom.

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

Question 2Medium

B: two liquids that mix completely and have different boiling points are separated by fractional distillation. The mixture is heated and the liquid with the lower boiling point, J (56 °C), vaporises first, so it is the one that is condensed and collected first. A: fractional distillation is right, but K has the higher boiling point, so it is collected after J. C, D: paper chromatography separates small amounts of dissolved substances, usually coloured ones; it does not give a sample of each liquid, so it is not used to separate two liquids that mix completely.

Textbook: Discover Chemistry GCE 'O' Level Science and Normal (A) Level Science Textbook, 3rd Edition (Marshall Cavendish Education, 2023), pp. 8–18

Question 3Easy

D: X is the nucleus, which contains the protons and neutrons (the nucleons); Y is an electron shell (energy level), and the electrons are arranged in shells around the nucleus. A: the two regions are swapped. B, C: electrons are found in the shells, not in the nucleus; B also puts the protons in a shell.

Textbook: Discover Chemistry GCE 'O' Level Science and Normal (A) Level Science Textbook, 3rd Edition (Marshall Cavendish Education, 2023), pp. 38, 44–45

Question 4Medium

A: each magnesium atom (2.8.2) loses its 2 outer electrons to form Mg²⁺, whose outer shell is now a full shell of 8; each oxygen atom (2.6) gains these 2 electrons to form O²⁻ with 8 outer electrons (6 dots and 2 crosses). B: the magnesium ion is drawn with its 2 outer electrons still present, which does not agree with the charge 2+. C: the oxide ion is drawn with only its own 6 electrons, so the 2 electrons it gained are missing and it could not be 2−. D: the charges are the wrong way round; the metal loses electrons to become positive and the non-metal gains them to become negative.

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

Question 5Hard

C: three sulfate ions carry a total charge of 6−, shared by two M ions, so each M ion is M³⁺. The oxide ion is O²⁻; two M³⁺ (6+) are balanced by three O²⁻ (6−), giving M₂O₃. A: this would need M²⁺. B: this would need M⁺. D: the charge numbers (3 and 2) have been used directly as the subscripts instead of being crossed over.

Textbook: Discover Chemistry GCE 'O' Level Science and Normal (A) Level Science Textbook, 3rd Edition (Marshall Cavendish Education, 2023), pp. 94–97

Question 6Medium

B: one mole of any substance contains the Avogadro constant, 6.02 × 10²³, of its particles; for water the particles are molecules. A: this counts the atoms, but each molecule has 3 atoms (2 H and 1 O), so one mole of water has 3 × 6.02 × 10²³ atoms. C: the mass of one mole of water is its relative molecular mass in grams, 18 g. D: 24 dm³ is the volume of one mole of a gas at r.t.p.; water is a liquid at r.t.p.

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

Question 7Medium

B: statements 1 and 3 are correct. A base reacts with an acid to form a salt and water (calcium chloride and water here), and a base reacts with an ammonium salt on heating to give ammonia, a salt and water: Ca(OH)₂ + 2NH₄Cl → CaCl₂ + 2H₂O + 2NH₃. Statements 2 and 4 describe reactions of acids (with a reactive metal, and with a carbonate), not of bases: magnesium does not react with calcium hydroxide, and no carbon dioxide forms with sodium carbonate. So A, C and D each include at least one of these wrong statements.

Textbook: Discover Chemistry GCE 'O' Level Science and Normal (A) Level Science Textbook, 3rd Edition (Marshall Cavendish Education, 2023), pp. 127–129

Question 8Medium

B: 2Mg + CO₂ → 2MgO + C. The magnesium gains oxygen (it becomes magnesium oxide), and gaining oxygen is oxidation. At the same time the carbon dioxide loses oxygen (it becomes carbon), which is reduction. A: gaining oxygen is oxidation, not reduction. C: losing oxygen is reduction, not oxidation. D: the carbon dioxide loses oxygen; it does not gain any.

Textbook: Discover Chemistry GCE 'O' Level Science and Normal (A) Level Science Textbook, 3rd Edition (Marshall Cavendish Education, 2023), pp. 155–156

Question 9Medium

A: elements in a period change from metals on the left to non-metals on the right. P (proton number 12, magnesium) is near the left of Period 3 and is a metal; Q (proton number 16, sulfur) is near the right and is a non-metal. B: the order is reversed; the metallic character decreases from left to right, so the element with the larger proton number is the more non-metallic. C, D: Period 3 contains both kinds of element, and P and Q are on different sides of it, so one is a metal and the other a non-metal.

Textbook: Discover Chemistry GCE 'O' Level Science and Normal (A) Level Science Textbook, 3rd Edition (Marshall Cavendish Education, 2023), p. 173

Question 10Easy

C: the melting points of the Group 1 metals decrease down the group (lithium, sodium, potassium). Sodium is between lithium and potassium, so its melting point must be between 63 °C and 181 °C; 98 °C is the only value in this range. A, B: these are lower than the melting point of potassium, which is below sodium in the group. D: this is higher than the melting point of lithium, so it follows the wrong trend (melting points increasing down the group).

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

Question 11Easy

D: this atom has the electronic configuration 2.8: its outer shell is full. A full outer shell is a stable arrangement, so the atom has no tendency to lose, gain or share electrons, and the element (neon, a noble gas) is unreactive. A, B: atoms with 1 or 2 outer shell electrons (2.1 and 2.2) lose them easily, and so are reactive metals. C: an atom with 7 outer shell electrons (2.7) is one electron short of a full shell, so it gains an electron very readily and is very reactive.

Textbook: Discover Chemistry GCE 'O' Level Science and Normal (A) Level Science Textbook, 3rd Edition (Marshall Cavendish Education, 2023), p. 178

Question 12Medium

A: in experiment 2 the volume of hydrogen rises faster (30 cm³ in the first 20 s, against 18 cm³) and reaches its final value sooner (by 40 s, against 80 s), so the rate is greater. Both experiments end at 40 cm³: the same amount of acid is used, and the acid is the limiting reactant, so the same total volume of hydrogen forms. B: the total volume is the same, not greater; a faster rate does not make more product. C: a reaction that takes longer to finish is the slower one. D: the same final volume shows only that the same amount of product forms; the rate depends on how quickly it forms.

Textbook: Discover Chemistry GCE 'O' Level Science and Normal (A) Level Science Textbook, 3rd Edition (Marshall Cavendish Education, 2023), pp. 209–212

Question 13Medium

D: in fractional distillation the hydrocarbons with the largest molecules have the highest boiling points; they condense lowest down the column (R) while the hydrocarbons with the smallest molecules and the lowest boiling points travel to the top (P). A: P is the fraction with the lowest boiling points. B: crude oil is a mixture of many hydrocarbons, and each fraction is still a mixture, of hydrocarbons with similar boiling points. C: the fraction with the largest molecules (R) is the hardest to ignite; the fraction with the smallest molecules (P) is the easiest.

Textbook: Discover Chemistry GCE 'O' Level Science and Normal (A) Level Science Textbook, 3rd Edition (Marshall Cavendish Education, 2023), pp. 228–231

Question 14Medium

C: methane, an alkane, is saturated and generally unreactive. It burns (combustion), and in sunlight or ultraviolet light it reacts with chlorine in a substitution reaction, in which a hydrogen atom is replaced by a chlorine atom. A, B, D: addition and polymerisation are reactions of unsaturated hydrocarbons such as alkenes, because the C=C double bond has to open; methane has only single bonds.

Textbook: Discover Chemistry GCE 'O' Level Science and Normal (A) Level Science Textbook, 3rd Edition (Marshall Cavendish Education, 2023), pp. 244–245

Question 15Medium

D: cracking breaks large alkane molecules into smaller molecules and makes alkenes and hydrogen. An alkene is unsaturated, so it decolourises aqueous bromine (orange to colourless); hydrogen gives a ‘pop’ with a lighted splint. A, B: an alkane is saturated and does not decolourise aqueous bromine, so the test shows that an alkene must be present; an alkane may also be in the mixture, but nothing in the tests shows it. A, C: carbon dioxide does not burn; it would put out a lighted splint, and it forms only when a hydrocarbon burns, not when it is cracked.

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

Question 16Medium

A: poly means many; a polyunsaturated oil has more than one carbon–carbon double bond in each molecule. Because it has C=C double bonds it is unsaturated, so it turns aqueous bromine from orange to colourless. B: with more than one C=C bond the oil must decolourise aqueous bromine; there is no reaction only for a saturated compound. C: an oil whose molecules have only one C=C bond does decolourise aqueous bromine, but it is unsaturated, not polyunsaturated. D: an oil that does not change aqueous bromine has no C=C bonds, so it is saturated.

Textbook: Discover Chemistry GCE 'O' Level Science and Normal (A) Level Science Textbook, 3rd Edition (Marshall Cavendish Education, 2023), p. 254

Question 17Hard

D: ethanol is oxidised to ethanoic acid by acidified potassium manganate(VII) (method 3), and slowly by oxygen in the air (method 4), which is why an opened bottle of wine turns sour. Method 1 fails because dilute sulfuric acid is not an oxidising agent: acidifying alone changes nothing. Method 2 is complete combustion, which oxidises the ethanol all the way to carbon dioxide and water, not to ethanoic acid. So A, B and C each include method 1 or method 2.

Textbook: Discover Chemistry GCE 'O' Level Science and Normal (A) Level Science Textbook, 3rd Edition (Marshall Cavendish Education, 2023), p. 266

Question 18Easy

C: poly(ethene) is a typical plastic, used for bags and clingfilm because it is flexible (it can be folded and wrapped round things) and insoluble in water (it keeps its shape and keeps water out). A, B: a brittle material would crack and break when folded or stretched, so it could not be used for a bag or a wrapping film. B, D: a plastic that dissolved in water would be of no use for carrying or covering wet items; poly(ethene) is insoluble in water.

Textbook: Discover Chemistry GCE 'O' Level Science and Normal (A) Level Science Textbook, 3rd Edition (Marshall Cavendish Education, 2023), p. 279

Question 19Medium

A: dry air is about 78% nitrogen and 21% oxygen. The remaining 1% is mainly argon, the main noble gas, with a very small amount of carbon dioxide (about 0.04%), so argon is present in a larger volume than carbon dioxide. B: carbon dioxide and argon are in the wrong order. C: nitrogen and oxygen are in the wrong order; there is much more nitrogen (78%) than oxygen (21%). D: both pairs are in the wrong order.

Textbook: Discover Chemistry GCE 'O' Level Science and Normal (A) Level Science Textbook, 3rd Edition (Marshall Cavendish Education, 2023), pp. 290–291

Question 20Easy

C: lightning, like the spark, supplies enough energy for nitrogen and oxygen in the air to react, forming nitrogen oxides (the colourless nitrogen monoxide, which reacts with more oxygen to form brown nitrogen dioxide). A: carbon monoxide forms by the incomplete combustion of carbon-containing fuels, and there is no fuel in the flask. B: methane is formed by the decay of vegetation, not from the gases of dry air. D: sulfur dioxide comes from volcanoes and from burning fuels that contain sulfur; air contains no sulfur.

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

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