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

C: each coloured substance in a paint travels up the paper to its own height, so a paint that gives one spot contains only one coloured substance. Paint L gives one spot. A: paint J gives three spots, so it contains three coloured substances. B, D: paints K and M each give two spots, so each is a mixture of two coloured substances.

A: both types of atom have the same proton number (10), so they are atoms of the same element, but their nucleon numbers differ (20 and 22), so they have different numbers of neutrons (10 and 12). Atoms of the same element with different numbers of neutrons are isotopes. B, D: the proton number is the same (10) for both, because both are neon; atoms with different numbers of protons are atoms of different elements. C: the numbers of neutrons are different (10 and 12), and neutral atoms of the same element have the same number of electrons.

C: the proton number is the number at the bottom: 29 for copper and 30 for zinc, so Cu²⁺ has fewer protons. The number of neutrons is the nucleon number minus the proton number: 64 − 29 = 35 in Cu²⁺ and 65 − 30 = 35 in Zn²⁺, so the numbers of neutrons are the same. Each ion has lost two electrons from its atom, so Cu²⁺ has 29 − 2 = 27 electrons and Zn²⁺ has 30 − 2 = 28 electrons: fewer for Cu²⁺. A: the nucleon number is smaller for copper (64 against 65), but neutrons are nucleon number minus proton number, and the two ions have 35 neutrons each. B: both ions have a charge of 2+, but the atoms they came from have different numbers of electrons (29 and 30), so the ions still differ by one electron. D: copper and zinc are different elements with different proton numbers (29 and 30), so the numbers of protons are not the same.

B: sodium chloride is a giant lattice of oppositely charged ions. In the solid the ions are held in fixed positions, so no charge can flow and the lamp does not light. When it is molten, the lattice has broken down and the ions are free to move; when it is dissolved in water, the ions are also separated and free to move. In both cases the moving ions carry the current, so the lamp lights. A: this leaves out the solution, in which the ions are also free to move. C: this leaves out the molten compound, in which the ions are free to move too. D: the ions in solid sodium chloride are held in fixed positions by strong electrostatic forces, so the solid does not conduct.

D: an electric current in a liquid needs charged particles that are free to move (ions) or free electrons. Propanone is made of neutral molecules, so it has neither and does not conduct. A: the covalent bonds within a molecule are strong, but this says nothing about charged particles. B: weak forces between molecules explain the low melting and boiling points of propanone, not its lack of conductivity. C: how closely the molecules are packed does not decide whether a substance conducts; neutral molecules cannot carry a current however close they are.

B: a green precipitate with aqueous sodium hydroxide shows that the solution contains iron(II) ions, Fe²⁺; iron(III) ions give a red-brown precipitate. The Fe²⁺ ions react with hydroxide ions, OH⁻, to form solid iron(II) hydroxide. One Fe²⁺ ion needs two OH⁻ ions so that the charges balance (2+ and 2 × 1−), giving the formula Fe(OH)₂. A precipitate is a solid, so its state symbol is (s). The sodium ions in the sodium hydroxide are spectator ions and are left out of an ionic equation. A: this is the equation for iron(III) ions, which form a red-brown precipitate, not a green one. C: the equation balances, but a precipitate is a solid, (s), not (aq). D: only one OH⁻ ion is shown, so the equation is not balanced (the net charge on the left is 2+ plus 1− = 1+, but Fe(OH)₂ has no charge).

B: amount of Mg = 0.60 ÷ 24 = 0.025 mol. The equation shows that 2 mol of Mg react with 1 mol of O₂, so the amount of O₂ = 0.025 ÷ 2 = 0.0125 mol. Volume = 0.0125 × 24 000 = 300 cm³ at r.t.p. A: 150 cm³ halves the amount of oxygen a second time. C: 600 cm³ is the volume for 0.025 mol of oxygen: the 2 : 1 ratio in the equation has been ignored. D: 1200 cm³ doubles the amount of magnesium instead of halving it.

C: a lower pH means a higher concentration of hydrogen ions. The conditioner (pH 4) is acidic and has more H⁺ ions than the hand soap (pH 10), which is alkaline, so the hand soap has the lower concentration of H⁺ ions. A: a solution with a pH below 7 has more hydrogen ions than hydroxide ions, not fewer. B: every aqueous solution contains some hydrogen ions; in an alkaline solution there are fewer H⁺ ions than OH⁻ ions, but not none. D: the concentration of hydroxide ions rises as the pH rises, so it is higher in the hand soap (pH 10) than in the conditioner (pH 4).

D: copper(II) oxide is the oxide of a metal and reacts with acids only, so it is a basic oxide; nitrogen dioxide is a non-metal oxide that dissolves in water to form an acidic solution (it is one cause of acid rain), so it is acidic; nitrogen monoxide is a non-metal oxide that is insoluble in water and reacts with neither acids nor alkalis, so it is neutral; zinc oxide reacts with both acids and alkalis, so it is amphoteric. A: nitrogen dioxide is acidic, not neutral, and nitrogen monoxide is neutral, not acidic: the two have been swapped. B: nitrogen monoxide is neutral, not acidic; it does not dissolve in water to form an acid. C: copper(II) oxide reacts only with acids, so it is basic, not amphoteric, and zinc oxide reacts with alkalis as well as acids, so it is amphoteric, not basic.

A: aluminium ions form a white precipitate with both aqueous sodium hydroxide and aqueous ammonia. The precipitate dissolves in excess sodium hydroxide but not in excess ammonia. B: calcium ions form a white precipitate with sodium hydroxide that does not dissolve in excess, and give no precipitate (or only a trace) with aqueous ammonia. C: ammonium ions give no precipitate with either reagent; ammonia gas is given off when the mixture with sodium hydroxide is warmed. D: zinc ions also give a white precipitate with both reagents, but it dissolves in excess ammonia as well as in excess sodium hydroxide.

B: an oxidising agent oxidises the colourless iodide ions in aqueous potassium iodide to iodine, so the solution turns from colourless to brown. A: the colour change is the reverse of this; the solution starts colourless, not brown. C, D: purple is the colour of acidified potassium manganate(VII), which is the test for a reducing agent, and it turns from purple to colourless in that test.

C: element Y has 7 electrons in its outer shell. Atoms with many outer electrons (usually 4 or more) gain electrons to fill the shell, which is typical of a non-metal. A, B, D: elements W, X and Z have 1, 2 and 3 outer electrons. Atoms with few outer electrons lose them easily to form positive ions, which is typical of a metal.

D: bromine is a red-brown liquid at room temperature. Down Group 17 the colour gets darker and the state changes from gas (chlorine) to liquid (bromine) to solid (iodine). A: purple-black solid is iodine. B: yellow-green gas is chlorine. C: bromine has the right colour, but it is a liquid, not a gas, at room temperature.

A: an endothermic process takes in energy from the surroundings. Melting a solid needs energy to separate the particles from their fixed positions, so energy is taken in. B: molten metal setting is freezing, which gives out energy to the surroundings. C: burning is an exothermic reaction; it gives out energy as heat and light. D: condensing gives out energy to the surroundings as the particles come together.

A: in the first 20 s, 30 cm³ of hydrogen is collected, an average rate of 30 ÷ 20 = 1.5 cm³ / s. Between 60 s and 80 s the volume rises from 60 cm³ to 70 cm³, an average rate of 10 ÷ 20 = 0.5 cm³ / s. 1.5 is three times 0.5. B: the rate is greatest at the start, where the curve is steepest; between 20 s and 40 s only 20 cm³ is collected (1.0 cm³ / s), less than the 1.5 cm³ / s in the first 20 s. C: the volume is still rising between 60 s and 80 s (60 to 70 cm³); the reaction stops at 80 s, when the curve becomes flat. D: 70 cm³ of hydrogen is collected in the first 80 s, so the average rate is 70 ÷ 80 = 0.875 cm³ / s, not 1.0 cm³ / s.

C: statements 1, 3 and 4 are correct. Wax is a larger molecule than the molecules in petrol, so it has more carbon atoms (1). Crude oil is a mixture of hydrocarbons with different boiling points, which are separated by fractional distillation (4). A fraction that is burned for energy is no longer available as a source of chemicals, so the two uses compete (3). Statement 2 is not correct: larger molecules have stronger forces between them and higher boiling points, so the wax fraction condenses at a higher temperature, lower down the column, than petrol. A, B, D each include statement 2, or leave out a correct statement.

B: alkenes form a homologous series of unsaturated hydrocarbons with the general formula C_nH_2n. Each CH₂ unit has a mass of 14, so n = 70 ÷ 14 = 5 and X is C₅H₁₀. A: C₅H₈ (M_r 68) does not fit C_nH_2n. C: C₅H₁₂ (M_r 72) fits C_nH_2n+2, the general formula of an alkane (a saturated hydrocarbon), not an alkene. D: C₆H₁₂ fits C_nH_2n but has M_r 84.

D: the vegetable oil is unsaturated (it has C=C double bonds), so it decolourises aqueous bromine. Hydrogen adds across the double bonds, forming a saturated solid fat with no C=C bonds, which does not decolourise aqueous bromine. A: the solid fat has no C=C bonds left, so it does not decolourise aqueous bromine. B: the oil still has C=C bonds, so it does decolourise aqueous bromine. C: the results are the wrong way round.

A: ethanol is oxidised to ethanoic acid by the oxidising agent acidified potassium manganate(VII). The manganate(VII) ions are reduced, so the purple solution turns colourless. B: the colour change is the wrong way round: purple manganate(VII) is used up, so it cannot appear. C, D: forming ethanoic acid (which has more oxygen than ethanol) is oxidation, not reduction.

C: statement 1 is about the effect on the surroundings (less waste in landfill), so it is an environmental issue. Statement 2 is about the cost of the process, so it is an economic issue. Statement 3 is about how people in a community take part and work together, so it is a social issue. A, B, D each match at least one statement with the wrong type of issue: for example, the cost in statement 2 is not an environmental or social issue, and the waste in statement 1 is not an economic or social issue.