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school. “Paper 42” means Paper 4, variant 2.
Practice questions
Original questions written by HomiLabs in the style of the exam, with answers and mark schemes. They are not
Cambridge past paper questions and have not been reviewed by Cambridge. Try each one before opening the answer.
Q1Ions & ionic bonding · Core · Paper 1 style · 1 mark
(a)Magnesium forms Mg²⁺ ions and nitrogen forms N³⁻ ions. What is the formula of magnesium nitride?[1]
Show answer
(a) A Mg₃N₂
Charges must balance: 3 × (+2) = +6 and 2 × (−3) = −6, so Mg₃N₂. Mg₂N₃ (B) simply swaps the numbers the wrong way and gives +4 and −9.
Q2Ions & ionic bonding · Core · Paper 2 style · 4 marks
Sodium chloride is obtained from rock salt mined at Khewra in Pakistan.
(a)The electronic configuration of a sodium atom is 2,8,1 and of a chlorine atom is 2,8,7. Describe what happens to the electrons when sodium reacts with chlorine to form sodium chloride.[2]
(b)Give the electronic configuration of the sodium ion and of the chloride ion.[2]
Show answer
(a) Each sodium atom transfers (loses) its one outer electron to a chlorine atom, which gains it; Na⁺ and Cl⁻ ions form.
B1 one electron transferred from Na to Cl; B1 Na⁺ and Cl⁻ ions formed (both with full outer shells).
(b) Na⁺: 2,8 Cl⁻: 2,8,8
B1 each.
Q3Ions & ionic bonding · Standard · Paper 2 style · 6 marks
Calcium oxide (quicklime) is made in lime kilns and used in cement and to treat acidic soil.
(a)Describe, in words, the dot-and-cross diagram for calcium oxide. Show the outer-shell electrons and the charges on the ions.[3]
(b)Calcium oxide melts at about 2600 °C. Explain why it has such a high melting point.[2]
(c)Sodium chloride melts at 801 °C. Suggest why calcium oxide has a much higher melting point than sodium chloride.[1]
Show answer
(a) Calcium transfers two electrons to oxygen. Ca²⁺ has 8 electrons in its outer shell (2,8,8) and O²⁻ has 8 electrons in its outer shell (2,8), shown as 6 dots + 2 crosses; charges 2+ and 2−.
B1 two electrons transferred from Ca to O; B1 both ions shown with 8 outer electrons (O²⁻: 6 of its own + 2 from Ca); B1 charges Ca²⁺ and O²⁻.
(b) It is a giant ionic lattice with strong electrostatic attraction between oppositely charged ions, so a lot of energy is needed to overcome these forces.
B1 giant lattice / many strong electrostatic forces between oppositely charged ions; B1 much energy needed to overcome them.
(c) The ions in calcium oxide have larger charges (2+ and 2−) so the attraction between them is stronger.
B1 higher charges on ions → stronger attraction.
Q4Ions & ionic bonding · Standard · Paper 2 style · 4 marks
(a)Use the ions K⁺, Al³⁺, Fe³⁺, O²⁻, OH⁻, NO₃⁻ and SO₄²⁻ to write the formulae of:(i) potassium oxide(ii) aluminium sulfate(iii) iron(III) hydroxide(iv) aluminium nitrate[4]
Show answer
(a) (i) K₂O (ii) Al₂(SO₄)₃ (iii) Fe(OH)₃ (iv) Al(NO₃)₃
B1 each. Brackets are needed around SO₄, OH and NO₃ when there is more than one.
Q5Ions & ionic bonding · Standard · Paper 2 style · 4 marks
(a)Solid potassium bromide does not conduct electricity, but molten potassium bromide does. Explain this difference.[3]
(b)State one other condition in which potassium bromide conducts electricity.[1]
Show answer
(a) Conduction needs mobile charged particles. In the solid the ions are held in fixed positions in the lattice and cannot move; when molten the ions are free to move and carry charge.
B1 ions present in both; B1 ions fixed / cannot move in solid; B1 ions mobile in molten (or aqueous) state.
(b) When dissolved in water (aqueous solution).
B1.
Q6Ions & ionic bonding · Challenge · Paper 2 style · 5 marks
(a)Lithium (2,1) reacts with nitrogen (2,5) to form the ionic compound lithium nitride, Li₃N. Explain, in terms of electron transfer, why the formula is Li₃N.[3]
(b)State the number of protons and electrons in (i) a lithium ion, Li⁺ (ii) a nitride ion, N³⁻. (Proton numbers: Li = 3, N = 7.)[2]
Show answer
(a) Each lithium atom loses one electron to form Li⁺. A nitrogen atom needs to gain three electrons to form N³⁻ (2,8). So three lithium atoms are needed for each nitrogen atom.
B1 Li loses 1 e⁻ → Li⁺; B1 N gains 3 e⁻ → N³⁻; B1 three Li needed per N so charges balance.
(c) Both have 2,8,8, but the ion has only 16 protons and a 2− charge; it is attracted to positive ions and can lose electrons (be oxidised), whereas argon is a neutral atom.
B1 same configuration 2,8,8; B1 different number of protons / ion is charged, so attracts positive ions or can be oxidised.
Q8Atomic structure & isotopes · Core · Paper 1 style · 1 mark
(a)An atom has 11 protons and 12 neutrons. Which statement about this atom is correct?[1]
Show answer
(a) A Its nucleon number is 23 and it has 11 electrons.
Nucleon number = protons + neutrons = 11 + 12 = 23; a neutral atom has as many electrons as protons (11). C wrongly uses the neutron number for electrons.
Q9Atomic structure & isotopes · Core · Paper 2 style · 4 marks
(a)An atom of potassium is shown as ³⁹₁₉K. State the number of protons, neutrons and electrons in this atom.[3]
(b)Give the electronic configuration of the potassium atom.[1]
Show answer
(a) 19 protons, 20 neutrons, 19 electrons
B1 each; neutrons = 39 − 19 = 20.
(b) 2,8,8,1
B1.
Q10Atomic structure & isotopes · Core · Paper 2 style · 4 marks
(a)Chlorine has two isotopes, chlorine-35 and chlorine-37. State what is meant by the term isotopes.[2]
(b)Explain why both isotopes of chlorine have the same chemical properties.[1]
(c)State the number of neutrons in an atom of chlorine-37. (Proton number of Cl = 17.)[1]
Show answer
(a) Atoms of the same element with the same number of protons but different numbers of neutrons.
B1 same proton number / same element; B1 different number of neutrons (different nucleon number).
(b) They have the same number of electrons and the same electronic configuration (7 outer electrons).
B1.
(c) 20
B1: 37 − 17 = 20.
Q11Atomic structure & isotopes · Standard · Paper 2 style · 3 marks
Boron compounds are used to make heat-resistant glassware for laboratories.
(a)Boron has two isotopes: boron-10 (abundance 19.9 %) and boron-11 (abundance 80.1 %). Calculate the relative atomic mass of boron. Give your answer to 3 significant figures.[2]
(b)State the number of neutrons in each isotope. (Proton number of B = 5.)[1]
Q12Atomic structure & isotopes · Standard · Paper 2 style · 4 marks
(a)A sample of magnesium contains 79.0 % magnesium-24, 10.0 % magnesium-25 and 11.0 % magnesium-26. Calculate the relative atomic mass of magnesium to 3 significant figures.[2]
(b)State the numbers of protons, neutrons and electrons in the ion ²⁶Mg²⁺. (Proton number of Mg = 12.)[2]
Q13Atomic structure & isotopes · Challenge · Paper 2 style · 4 marks
(a)Gallium has two isotopes, gallium-69 and gallium-71. The relative atomic mass of gallium is 69.72. Calculate the percentage abundance of each isotope.[3]
(b)Explain why the relative atomic mass of gallium is closer to 69 than to 71.[1]
Q15Simple covalent molecules · Core · Paper 1 style · 1 mark
(a)Which molecule contains a triple covalent bond?[1]
Show answer
(a) B N₂
N₂ has N≡N (three shared pairs) so each N has 8 outer electrons. O₂ (A) has a double bond, Cl₂ a single bond, CH₄ four single bonds.
Q16Simple covalent molecules · Core · Paper 2 style · 5 marks
Natural gas, which is mainly methane, is piped to many homes for cooking.
(a)Describe the bonding in a molecule of methane, CH₄. Include the number of shared pairs and the number of outer-shell electrons around each atom.[3]
(b)Methane boils at −162 °C. Explain why its boiling point is so low.[2]
Show answer
(a) Carbon shares one pair of electrons with each of four hydrogen atoms: four single covalent bonds (4 shared pairs). Carbon then has 8 outer electrons and each hydrogen has 2.
B1 covalent bond = shared pair of electrons; B1 four shared pairs / single bonds; B1 C has 8 and each H has 2 outer electrons.
(b) Methane is a simple molecular substance; the intermolecular forces between molecules are weak, so little energy is needed to separate the molecules.
B1 weak intermolecular forces (not covalent bonds broken); B1 little energy needed.
Q17Simple covalent molecules · Standard · Paper 2 style · 4 marks
(a)Describe, in words, the dot-and-cross diagram for a molecule of carbon dioxide, CO₂. Show only outer-shell electrons.[3]
(b)Calculate the total number of electrons that are shared in one CO₂ molecule.[1]
Show answer
(a) Carbon in the centre forms a double bond (two shared pairs) with each oxygen atom. Each oxygen also has two non-bonding (lone) pairs. Every atom has 8 outer electrons.
B1 two double bonds / four shared pairs; B1 two lone pairs on each O; B1 8 electrons around each atom.
(b) 8
B1: 2 double bonds × 2 pairs × 2 electrons = 8.
Q18Simple covalent molecules · Standard · Paper 2 style · 4 marks
(a)Methanol has the structural formula CH₃OH. State the number of covalent bonds (bonding pairs) in one molecule and the number of non-bonding pairs on the oxygen atom.[2]
(b)Hydrogen chloride is a covalent gas. Explain why pure hydrogen chloride does not conduct electricity.[2]
Show answer
(a) 5 bonding pairs; 2 non-bonding pairs on oxygen
B1 5 (three C–H, one C–O, one O–H); B1 2 lone pairs (O has 6 outer electrons, 2 used in bonds).
(b) It is made of neutral molecules; there are no ions or delocalised electrons free to move and carry charge.
B1 molecules / no ions; B1 no mobile charged particles.
Q19Simple covalent molecules · Challenge · Paper 2 style · 6 marks
Melting and boiling points of three halogens:chlorine, Cl₂: m.p. −101 °C, b.p. −34 °Cbromine, Br₂: m.p. −7 °C, b.p. 59 °Ciodine, I₂: m.p. 114 °C, b.p. 184 °C
(a)State the physical state of each substance at 25 °C.[2]
(b)Explain the trend in boiling points from chlorine to iodine.[2]
(c)A student says 'when bromine boils, the Br–Br bonds break'. Explain why this is wrong.[2]
Show answer
(a) chlorine: gas; bromine: liquid; iodine: solid
2 marks for all three correct, 1 mark for two.
(b) The molecules get larger (more electrons), so the intermolecular forces get stronger and more energy is needed to separate the molecules.
B1 larger molecules down the group; B1 stronger intermolecular forces need more energy to overcome.
(c) Boiling only separates whole Br₂ molecules by overcoming weak intermolecular forces; the strong covalent Br–Br bond inside each molecule stays intact.
B1 intermolecular forces overcome; B1 covalent bonds within molecules not broken (Br₂ molecules remain).
Q20Metallic bonding · Core · Paper 2 style · 3 marks
(a)Describe the structure and bonding in a metal such as copper.[2]
(b)Explain why copper conducts electricity when solid.[1]
Show answer
(a) A giant lattice of positive metal ions in a 'sea' of delocalised electrons; the bonding is the electrostatic attraction between the positive ions and the delocalised electrons.
B1 lattice of positive ions; B1 delocalised electrons / attraction between ions and electrons.
(b) The delocalised electrons are free to move through the structure and carry charge.
B1.
Q21Metallic bonding · Standard · Paper 2 style · 3 marks
Electricians in Faisalabad use copper wire for house wiring.
(a)Copper is drawn into thin wires for electrical cables. Explain, in terms of structure, why metals can be bent and drawn into wires without breaking.[3]
Show answer
(a) The positive ions are arranged in layers. When a force is applied the layers slide over each other. The delocalised electrons move with them, so the metallic bonding is not broken.
B1 layers of ions; B1 layers slide over one another; B1 bonding remains because electrons are delocalised (attraction kept in new positions).
Q22Metallic bonding · Challenge · Paper 2 style · 5 marks
(a)Sodium melts at 98 °C and magnesium melts at 650 °C. Both are metals in Period 3. Suggest why magnesium has the higher melting point.[3]
(b)Explain why solid sodium chloride does not conduct electricity even though solid sodium does.[2]
Show answer
(a) Magnesium forms Mg²⁺ ions and gives two delocalised electrons per atom, while sodium forms Na⁺ and gives one. So the attraction between the positive ions and the delocalised electrons is stronger in magnesium and more energy is needed to break it.
B1 Mg²⁺ vs Na⁺ (higher charge); B1 more delocalised electrons per atom in Mg; B1 stronger metallic bonding / more energy needed.
(b) Sodium has delocalised electrons that can move; in solid sodium chloride the ions are fixed in the lattice and there are no free electrons.
Q23Structure and properties compared · Standard · Paper 1 style · 1 mark
(a)The table gives properties of four substances.substance: melting point / conducts when solid / conducts when moltenP: 1710 °C / no / noQ: 801 °C / no / yesR: −95 °C / no / noS: 1085 °C / yes / yesWhich substance has a giant ionic structure?[1]
Show answer
(a) B Q
Q: high melting point, conducts only when molten (ions become mobile). P (A) has a high melting point but never conducts, so it is giant covalent.
Q24Structure and properties compared · Standard · Paper 2 style · 6 marks
A student in Peshawar tests four substances.W: m.p. 660 °C; conducts as a solid and when molten; insoluble in waterX: m.p. 80 °C; does not conduct as a solid or when molten; insoluble in waterY: m.p. 770 °C; does not conduct as a solid; conducts when molten and when dissolved in waterZ: m.p. 3550 °C; does not conduct as a solid; insoluble in water
(a)Deduce the type of structure of each of W, X, Y and Z. Choose from: giant ionic, giant covalent, simple molecular, metallic.[4]
(b)Explain why Y conducts electricity when dissolved in water but not when solid.[2]
B1 each. W conducts as solid; X low m.p. and non-conductor; Y conducts only molten/dissolved; Z very high m.p., insoluble, non-conductor.
(b) In the solid the ions are fixed in the lattice; in solution the ions are free to move and carry charge.
B1 each point.
Q25Structure and properties compared · Challenge · Paper 2 style · 6 marks
(a)Silicon(IV) oxide melts at about 1700 °C but carbon dioxide is a gas at room temperature. Both are oxides of Group IV elements. Explain the difference in terms of structure and bonding.[3]
(b)Graphite and copper both conduct electricity. Compare how the charge is carried in each.[2]
(c)Suggest why magnesium oxide is used to line furnaces but sodium chloride is not.[1]
Show answer
(a) SiO₂ is a giant covalent structure: melting requires breaking many strong covalent bonds, needing a lot of energy. CO₂ is simple molecular: only weak intermolecular forces between molecules must be overcome.
B1 SiO₂ giant covalent with many strong covalent bonds broken; B1 CO₂ simple molecules; B1 weak intermolecular forces need little energy.
(b) Both by delocalised electrons: in graphite each carbon atom has one delocalised electron, which moves along the layers; in copper the delocalised electrons move throughout the lattice of positive ions.
B1 graphite: delocalised electrons (one per C, move along layers); B1 copper: sea of delocalised electrons around positive ions.
(c) MgO has a much higher melting point (stronger attraction between 2+ and 2− ions), so it does not melt at furnace temperatures.
B1.
Q26Giant covalent structures · Core · Paper 1 style · 1 mark
(a)Which statement explains why diamond is used in drill tips for cutting rock?[1]
Show answer
(a) A Each carbon atom is covalently bonded to four other carbon atoms in a rigid giant structure.
Four strong covalent bonds per atom in a 3-D giant lattice make diamond very hard. B and D describe graphite.
Q27Giant covalent structures · Standard · Paper 2 style · 4 marks
Graphite is used as a dry lubricant in locks and as electrodes in electrolysis cells.
(a)Describe the structure of graphite.[2]
(b)Use the structure to explain why graphite (i) is used as a lubricant (ii) is used for electrodes.[2]
Show answer
(a) Layers of carbon atoms arranged in hexagons; each carbon is covalently bonded to three others; weak forces between the layers.
B1 layers of hexagonal rings, each C bonded to 3 others; B1 weak forces between layers.
(b) (i) The layers can slide over each other because forces between layers are weak. (ii) Each carbon has one delocalised electron that can move and carry charge.
B1 each.
Q28Giant covalent structures · Challenge · Paper 2 style · 5 marks
(a)Describe the structure of silicon(IV) oxide, SiO₂.[2]
(b)Use your answer to (a) to show that the formula is SiO₂.[1]
(c)Sand (mainly silicon(IV) oxide) is used to make glass. Explain why sand does not conduct electricity and has a high melting point.[2]
Show answer
(a) A giant covalent lattice in which each silicon atom is covalently bonded to four oxygen atoms and each oxygen atom is bonded to two silicon atoms.
B1 giant covalent / 3-D lattice; B1 each Si bonded to 4 O and each O to 2 Si.
(b) Each Si has 4 bonds to O, but each O is shared between 2 Si, so each Si 'owns' 4 ÷ 2 = 2 O atoms; ratio Si : O = 1 : 2.
B1 correct reasoning.
(c) No ions or delocalised electrons are free to move; many strong covalent bonds must be broken to melt it, needing much energy.
B1 no mobile charged particles; B1 many strong covalent bonds.
Q29Elements, compounds & mixtures · Core · Paper 1 style · 1 mark
(a)Which of these is a mixture?[1]
Show answer
(a) A air
Air contains nitrogen, oxygen and other gases not chemically combined, in variable proportions. Water (B) is a compound with a fixed composition.
Q30Elements, compounds & mixtures · Standard · Paper 2 style · 5 marks
(a)Classify each of the following as an element, a compound or a mixture: nitrogen, glucose, sea water, magnesium oxide, mercury, brass.[3]
(b)Give two differences between a compound and a mixture.[2]
3 marks for all six correct; 2 marks for 4–5; 1 mark for 2–3.
(b) A compound has a fixed composition and its elements are chemically bonded (can only be separated by chemical reactions); a mixture has variable composition, its components are not chemically bonded and can be separated by physical methods. A compound's properties differ from those of its elements.
1 mark each for any two of: fixed vs variable composition; chemically combined vs not; separated by chemical vs physical means; properties of compound different from its elements.
Similar questions
Questions that test the same subtopic in different years, grouped so you can work on one skill at a time.
References only: find each question in its paper (official link where available, otherwise through your school).
