CHEMISTRY – Paper 1 Multiple Choice – mark scheme

C: the dry mixture is heated first, so the ammonium chloride sublimes and is collected as a solid on a cold surface; the residue (sodium chloride and sand) is stirred with water, the sand is filtered off, and the filtrate is evaporated until crystals of sodium chloride form. Each substance is obtained separately. A: water dissolves both salts, so filtering removes only the sand, and evaporating the filtrate leaves sodium chloride and ammonium chloride mixed together. B: the sand is never separated from the sodium chloride; evaporating the mixture of sand and sodium chloride solution leaves the two solids together again. D: the ammonium chloride has already dissolved in the water, so it passes through the filter paper into the filtrate; the residue is only sand, and heating it removes nothing.

B: statements 1, 2 and 4 are correct. Impurities can be harmful (1) or can change the properties of a substance, such as its colour (2), and measuring purity lets the maker check that the pigment is free from harmful impurities before it goes into the make-up (4). Statement 3 is not correct: even a very small amount of a toxic impurity (for example, a compound of a poisonous metal) can harm the person who uses the make-up, which is why purity must be measured and not assumed. A, C and D each include statement 3.

D: statements 2, 3 and 4 are correct. Perfume particles move randomly and keep colliding with air particles, so each follows a zig-zag path and the smell takes minutes to reach the far side (statement 2); because there are many more particles near the corner, more of them move away from the corner than move back, so the perfume spreads from where it is crowded to where there is little of it (statement 3); and when the perfume is evenly spread the particles do not stop: they keep moving and colliding, but there is no longer any overall movement in one direction (statement 4). Statement 1 is wrong: diffusion needs no air currents, because the particles move on their own even when the air is still. A, B and C each include statement 1.

A: isotopes are atoms of the same element with the same number of protons but different numbers of neutrons. Carbon-12 has 6 protons and 6 neutrons (12 − 6); carbon-14 has 6 protons and 8 neutrons (14 − 6), so it has two more neutrons. Both atoms have 6 electrons, because both have 6 protons and are neutral. B: the number of electrons is the same; it always equals the number of protons in a neutral atom. C: the number of protons is the same in both isotopes (atoms with a different number of protons would be atoms of a different element), and so is the number of electrons; only the neutrons differ. D: this has the protons and the neutrons the wrong way round; the extra mass of carbon-14 is due to the extra neutrons, not extra protons.

C: sodium chloride is an ionic compound. A crystal is a giant lattice of positive sodium ions and negative chloride ions in a regular pattern, with every ion held by strong electrostatic attraction to the oppositely charged ions around it. A: this shows positive ions in a sea of delocalised electrons, which is the structure of a metal. B: this shows separate pairs of ions, as if sodium chloride were made of small molecules; the ions are not in a giant lattice and are not joined to all of their neighbours. D: this shows atoms joined by covalent bonds in a giant structure, as in diamond; sodium chloride has no covalent bonds.

B: carbon dioxide is a simple molecular substance. When the solid changes to a gas, only the weak forces between the molecules are overcome, which takes very little energy, so this happens at a very low temperature. The covalent bonds inside the molecules are strong and stay intact. A: the covalent bonds inside a molecule are strong; they are not broken when carbon dioxide changes to a gas. C: carbon dioxide is a covalent substance and contains molecules, not ions. D: carbon dioxide does not have a giant covalent structure; a giant covalent structure would have a very high melting point.

A: gold is a lattice of positive ions in a sea of delocalised electrons. When it is hammered, layers of ions slide over each other while the electrons still hold the ions together, so gold bends instead of breaking; the delocalised electrons are free to move and carry the current, so gold conducts. B and D: the positive ions are fixed in the lattice and only vibrate; it is the electrons that move. C and D: metallic bonding is strong, not weak (metals have high melting points); gold can be hammered into shape because the layers can slide, not because the bonding is weak.

C: statements 1, 3 and 4 are correct. An alloy is a mixture of a metal with another element, so sterling silver is a mixture and not a compound (statement 1); the copper atoms are a different size from the silver atoms, so the regular layers are disturbed and cannot slide over each other as easily, which makes the alloy harder (statement 3); and the atoms are still held together by metallic bonding (statement 4). Statement 2 is wrong: the atoms in an alloy are not joined by covalent bonds; the copper atoms simply take the place of some silver atoms in the metallic lattice. A, B and D each include statement 2.

B: in graphite the carbon atoms are in layers held to each other by weak forces, so the layers slide off onto the paper; each carbon atom forms three bonds, leaving one electron that is delocalised and free to move along the layers, so graphite conducts; in diamond each carbon atom forms four strong covalent bonds to other carbon atoms in a giant structure, so it is very hard. A: the covalent bonds in the layers are strong; it is the weak forces between the layers that let them slide. C: graphite is not ionic; there are no ions, only delocalised electrons, that carry the current. D: diamond has no delocalised electrons (all four outer electrons of each carbon atom are in covalent bonds), which is why it does not conduct; its hardness is due to the strong covalent bonds in all directions.

D: M_r is the sum of the relative atomic masses of all the atoms in the formula. Al₂(SO₄)₃ contains 2 Al, 3 S and 12 O: (2 × 27) + (3 × 32) + (12 × 16) = 54 + 96 + 192 = 342. A: the 3 outside the bracket was ignored, so only one sulfate group was counted (54 + 96). B: the 3 was applied to the sulfur only (54 + 96 + 64 = 214); the oxygen atoms in the bracket must be multiplied by 3 as well. C: only one aluminium atom was counted (27 + 288 = 315), but there are two.

C: in 100 g of the acid there are 40.0 / 12 = 3.33 mol of C, 6.7 / 1 = 6.7 mol of H and 53.3 / 16 = 3.33 mol of O, a ratio of 1 : 2 : 1, so the empirical formula is CH₂O (mass 12 + 2 + 16 = 30). The molecular formula is a whole-number multiple of this: 90 / 30 = 3, so it is C₃H₆O₃. A: this is only the empirical formula; its mass is 30, not 90. B: this has a mass of 60, so it is twice the empirical formula, not three times. D: this has a mass of 180, which is six times the empirical formula.

B: moles of NaOH = 4.0 / 40 = 0.10 mol; the volume is 500 cm³ = 0.500 dm³; concentration = 0.10 / 0.500 = 0.20 mol / dm³. A: 0.10 is the number of moles, not the concentration; the volume was not used. C: the volume was converted wrongly (500 cm³ taken as 0.050 dm³): 0.10 / 0.050 = 2.0. D: 8.0 is the concentration in g / dm³ (4.0 g / 0.500 dm³); the mass was not converted to moles.

A: a weak acid is only partly ionised in water, so most of its particles remain as unchanged HA molecules and only a few have split into H⁺ ions and A⁻ ions (equal numbers of each). B: here every HA molecule has ionised, which is how a strong acid behaves. C: here no HA molecule has ionised, so the solution would contain no H⁺ ions and would not be acidic. D: this has more H⁺ ions than A⁻ ions; ionisation of HA always produces equal numbers of the two ions, so the diagram is impossible.

A: statements 1, 2 and 3 are correct. Calcium hydroxide is a base: its hydroxide ions react with the hydrogen ions in the soil (H⁺ + OH⁻ → H₂O), so the concentration of hydrogen ions falls and the pH rises (statements 1 and 2). If much more calcium hydroxide were added than the acid could use up, hydroxide ions would be left over and the soil would become alkaline (statement 3), which is why the amount added must be controlled. Statement 4 is wrong: calcium hydroxide adds hydroxide ions to the soil, so their concentration increases. B, C and D each include statement 4.

C: calcium oxide is the oxide of a metal, so it is basic (it reacts with water to give calcium hydroxide, an alkali, and with acids to give a salt and water); carbon dioxide is the oxide of a non-metal, so it is acidic (it reacts with calcium hydroxide, a base, to form calcium carbonate and water). A: the types are reversed. B: calcium oxide does not react with alkalis, so it is not amphoteric, and the oxide of a non-metal such as carbon is not amphoteric. D: carbon dioxide is not neutral; it dissolves in water to form an acidic solution and reacts with alkalis. (Carbon monoxide is the neutral oxide of carbon.)

B: iron is above hydrogen in the reactivity series, so it reacts with dilute sulfuric acid to form iron(II) sulfate and hydrogen. Iron is added in excess so that all the acid reacts; the unreacted iron is filtered off and the filtrate is crystallised. A: copper is below hydrogen in the reactivity series and does not react with dilute sulfuric acid. C: iron reacts with dilute hydrochloric acid, but the salt formed is iron(II) chloride, not iron(II) sulfate. D: iron(II) chloride, sodium sulfate, iron(II) sulfate and sodium chloride are all soluble, so no solid forms and nothing can be filtered off.

D: if carbonate ions are present they would also form a white precipitate (silver carbonate) with silver nitrate and give a false positive. The dilute nitric acid first reacts with any carbonate ions and removes them (as carbon dioxide and water), so any precipitate that then forms is silver chloride. A: silver nitrate is soluble in water and does not need acid to dissolve. B: the chloride ions must stay in solution as chloride ions to react with the silver ions. C: silver chloride is insoluble in water, with or without acid, so nothing needs to be done to stop it dissolving; the white precipitate is the positive result of the test.

C: in Ag₂S the oxidation state of silver is +1, so silver has lost electrons and its oxidation state has increased from 0 to +1; this is oxidation. A: the oxidation state of sulfur is −2 in both H₂S and Ag₂S, so sulfur is neither oxidised nor reduced. B: oxygen gains electrons (its oxidation state falls from 0 to −2), which is reduction, not oxidation. D: losing electrons is oxidation, not reduction.

D: electrolysis is the passing of an electric current through a molten or dissolved ionic compound, which causes chemical changes (decomposition) at the electrodes: zinc chloride is broken down into zinc and chlorine. A: electrons travel through the wires and the lamp; in the molten compound the current is carried by moving ions. B: the positive zinc ions go to the negative electrode and the negative chloride ions go to the positive electrode, so the two elements are formed at different electrodes. C: the ions are already present in zinc chloride (it is an ionic compound); melting frees them to move, which is why it conducts.

A: the solution contains Na⁺, H⁺, SO₄²⁻ and OH⁻ ions. At the negative electrode, sodium is more reactive than hydrogen, so hydrogen ions are discharged and hydrogen gas forms; at the positive electrode, sulfate ions are not discharged, so hydroxide ions are discharged instead and oxygen gas forms. B: sulfate ions are not discharged, and sulfur dioxide is not formed; hydroxide ions are discharged to give oxygen. C and D: sodium is above hydrogen in the reactivity series, so sodium ions stay in solution and sodium metal is not formed (D is also wrong at the positive electrode).

D: at the cathode, positive silver ions gain one electron each (reduction): Ag⁺(aq) + e⁻ → Ag(s). The oxygen at the anode comes from hydroxide ions of the water, which are discharged in preference to nitrate ions: 4OH⁻(aq) → 2H₂O(l) + O₂(g) + 4e⁻. Both equations balance in atoms and in charge (−4 on each side of the anode equation). A: the cathode equation has the electron on the wrong side, which would be oxidation, not the reduction of silver ions at the cathode. B: wrong cathode equation, and the anode equation does not balance in atoms: 2OH⁻ supplies only 2 oxygen atoms, but H₂O + O₂ contains 3 (the charges happen to balance, −2 on each side). C: the cathode equation is right, but the anode equation has only 2e⁻, so the charges do not balance; four electrons are lost for each O₂ molecule formed.

A: hydrogen for a fuel cell can be derived from water (for example by electrolysis) or from hydrocarbons such as methane (statements 1 and 2), and in the fuel cell it reacts with oxygen to generate electricity directly, without burning (statement 3). Statement 4 is incorrect: the fuel cell reacts hydrogen with oxygen, 2H₂ + O₂ → 2H₂O, so the only product is water. No carbon is present in the reactants, so no carbon dioxide forms. B, C and D each include statement 4.

B: an aluminium atom loses its 3 outer electrons to form an Al³⁺ ion; an oxygen atom gains 2 electrons to complete its outer shell of 8, forming an O²⁻ ion. For the compound to be neutral the charges must cancel: 2 × (+3) = 3 × (−2), so there are 2 aluminium ions to 3 oxide ions, Al₂O₃. A: this ignores the ionic charges (1 : 1 ratio). C: the ratio is the wrong way round, so the compound would not be neutral (3 × 3+ against 2 × 2−). D: this takes the number of outer electrons of oxygen (6) as its ionic charge; oxygen needs to gain only 2 electrons.

D: atoms of non-metals have four or more outer electrons (here 7). They complete their outer shell by gaining electrons, so they form negative ions. A: the description is correct for an atom with two outer electrons, but that is an atom of a metal (such as magnesium), not a non-metal. B: an atom with three outer electrons is an atom of a metal (such as aluminium); it loses those three electrons rather than gaining five. C: an atom with six outer electrons gains two electrons to reach eight; it does not lose six.

C: a substance is a gas if 25 °C is above its boiling point, a liquid if 25 °C is between its melting point and boiling point, and a solid if 25 °C is below its melting point. Chlorine (boiling point −34 °C) is a gas, bromine (melting point −7 °C, boiling point 59 °C) is a liquid and iodine (melting point 114 °C) is a solid. The states change from gas to liquid to solid down Group 17 because the melting and boiling points rise. A: bromine is above its melting point and below its boiling point, so it is a liquid, not a gas. B: iodine does not melt until 114 °C, so it is a solid, not a liquid. D: chlorine boils at −34 °C, so at 25 °C it is a gas, not a liquid.

B: the order of reactivity, most reactive first, is zinc, iron, copper, silver. Zinc and iron react with dilute hydrochloric acid, zinc more vigorously than iron; copper and silver do not react with the acid, and copper is the more reactive of the two. Increasing reactivity therefore runs silver, copper, iron, zinc. A: silver and copper are the wrong way round. C: iron and zinc are the wrong way round; zinc reacts more readily than iron. D: this is the order of decreasing reactivity.

A: the more reactive a metal, the more firmly it is combined in its compounds and the harder it is to obtain. Magnesium is high in the reactivity series (above carbon), so carbon cannot remove oxygen from magnesium oxide and magnesium has to be obtained by electrolysis. Zinc and copper are lower in the reactivity series (below carbon), so carbon can reduce their oxides when they are heated together. B: zinc is below carbon, so its oxide can be reduced by heating with carbon and it does not need electrolysis. C: magnesium is above carbon in the reactivity series, so heating its oxide with carbon does not produce magnesium. D: copper is a very unreactive metal, much lower than zinc; heating its oxide with carbon is enough, so electrolysis is not needed.

B: the activation energy is the minimum energy needed to start the reaction, the rise in energy from the reactants to the top of the peak (arrow X). The enthalpy change is the difference in energy between the products and the reactants (arrow Y); the products are lower than the reactants, which is why the mixture becomes warm and ΔH is negative. A: Z is the rise from the products to the peak, the activation energy of the reverse reaction, and not the enthalpy change. C: the two arrows are swapped. D: Z is the energy needed to go from products to the peak, not the energy needed to start the reaction from the reactants.

C: platinum is a catalyst here. A catalyst makes the reaction faster, so the oxygen forms more quickly, but it does not change the amount of hydrogen peroxide that decomposes, so the total volume of oxygen formed is the same. A, B: a catalyst does not make more product; the amount of oxygen depends only on the amount of hydrogen peroxide used. A, D: a catalyst increases the rate of the reaction, so the rate is not the same.

D: only the temperature should be changed, so the size of the fizzer (statement 3) and the volume of water must be kept the same, and the rate must be judged by something that depends on how fast the reaction goes: the shorter the time taken for the fizzing to stop, the faster the reaction (statement 4). Statement 1 is incorrect: the same fizzer makes the same total volume of carbon dioxide at every temperature, so the total volume does not show how fast it was made. Statement 2 is incorrect: if the volume of water is different, any difference in the time could be due to the volume as well as the temperature, so it is not a fair test. A, B and C each include statement 1 or statement 2.

C: bioethanol is a biofuel. Sugarcane can be grown again, so more bioethanol can be made as it is used: it is a renewable alternative to non-renewable fuels such as natural gas and crude oil (statement 1). It is made by fermenting the sugar from the sugarcane (statement 3). Like other carbon-containing fuels it forms carbon dioxide and water when it burns completely, C₂H₅OH + 3O₂ → 2CO₂ + 3H₂O (statement 4); the sugarcane takes in carbon dioxide as it grows, but the gas is still given out when the fuel burns. Statement 2 is incorrect: bioethanol, C₂H₅OH, contains oxygen as well as carbon and hydrogen, so it is not a hydrocarbon. A leaves out statement 4, which is correct; B and D include statement 2.

A: members of a homologous series have similar chemical properties because they have the same functional group (statement 1), and their physical properties change gradually as the molecules get bigger: the boiling points rise steadily from −48 °C to 63 °C, so the next member is predicted to boil above 63 °C (statement 3). Statement 2 is incorrect: in every member the number of hydrogen atoms is twice the number of carbon atoms (C₃H₆, C₄H₈ ...), so the general formula is C_nH_2n; C_nH_2n+2 is the general formula of the alkanes. Statement 4 is incorrect: the next member after C₆H₁₂ has one more CH₂ group, C₇H₁₄. B, C and D each include statement 2 or statement 4.

D: methane is an alkane, a saturated hydrocarbon, so it does not react with aqueous bromine. It burns completely in air to form carbon dioxide and water, and in ultraviolet light it undergoes a substitution reaction with chlorine: CH₄ + Cl₂ → CH₃Cl + HCl. A: carbon monoxide burns to form carbon dioxide only; it contains no hydrogen, so no water forms. B: ethene is unsaturated and decolourises aqueous bromine. C: hydrogen burns to form water only; it contains no carbon.

A: cracking breaks a large hydrocarbon molecule into smaller molecules, including an alkene, and the equation must balance. C₁₄H₃₀ → C₁₀H₂₂ + 2C₂H₄ has 14 carbon atoms and 30 hydrogen atoms on each side, and C₂H₄ is an alkene. B: the right-hand side has 10 + 4 = 14 carbon atoms but 22 + 10 = 32 hydrogen atoms, so it does not balance; two alkanes can never be the only products of cracking. C: the right-hand side has 22 + 12 = 34 hydrogen atoms against 30 on the left, so it does not balance. D: this equation balances, but it is the wrong way round: small molecules are joining to form a larger one, which is the opposite of cracking.

A: ethene is unsaturated and bromine adds across the C=C double bond in an addition reaction, C₂H₄ + Br₂ → C₂H₄Br₂. The double bond becomes a single bond and one bromine atom bonds to each carbon atom, giving 1,2-dibromoethane. B: this has both bromine atoms on the same carbon atom; in the addition, one bromine atom goes to each of the two carbon atoms of the double bond. C: this is bromoethane, which has only one bromine atom; it is the product of ethene and hydrogen bromide, not of ethene and bromine. D: this still contains a C=C bond and has only one bromine atom, so it is not the product of an addition reaction.

C: the molecule contains the carboxylic acid group, –CO₂H, so it is a carboxylic acid. It has only one carbon atom, the one in the –CO₂H group, so it is methanoic acid. A: ethanoic acid has two carbon atoms, CH₃CO₂H. B, D: ethanol and methanol are alcohols; they contain the –OH group but no C=O group. Methanol has one carbon atom but is CH₃OH, and ethanol has two carbon atoms.

B: ethanoic acid is a weak acid, but like other acids it reacts with a carbonate to give a salt, water and carbon dioxide: CaCO₃ + 2CH₃CO₂H → (CH₃CO₂)₂Ca + H₂O + CO₂. The salt is calcium ethanoate. A calcium ion is Ca²⁺ and an ethanoate ion is CH₃CO₂⁻, so two ethanoate ions are needed to balance the charge: (CH₃CO₂)₂Ca. A, C: CH₃CO₂Ca does not balance in charge (1− against 2+). C, D: hydrogen is given off when an acid reacts with a reactive metal, not with a carbonate; here the gas is carbon dioxide.

C: nylon is a polyamide, a condensation polymer with amide links, and it is used to make man-made fibres for clothing, fishing line and parachutes. A, B: clingfilm is made from the plastic poly(ethene), not from a fibre such as nylon. B, D: a polyester is a different type of condensation polymer, with ester links; Terylene is the polyester used in clothing, curtains and sleeping bags.

B: P is carbon monoxide, CO, the very toxic gas formed by incomplete combustion of carbon-containing fuels. Q is nitrogen dioxide, NO₂: nitrogen oxides are formed from nitrogen and oxygen of the air in the hot engines of vehicles and by lightning, and nitrogen dioxide forms acid rain. R is sulfur dioxide, SO₂: it is formed when fossil fuels that contain sulfur, such as coal, burn and it is given out by volcanoes; it also forms acid rain. A: the formulae of Q and R are swapped; the sources of sulfur dioxide are burning coal and volcanoes, and the sources of nitrogen dioxide are vehicle engines and lightning. C: carbon monoxide is the pollutant formed when a fuel burns in too little air, so it is P, not Q. D: sulfur dioxide comes from sulfur in fuels and from volcanoes; it is not formed by burning in too little air, and carbon monoxide does not cause acid rain.

D: CFCs rise into the upper atmosphere (the stratosphere), where ultraviolet radiation breaks them down and releases chlorine atoms. The chlorine atoms react with ozone and break it down. The ozone layer absorbs harmful ultraviolet radiation from the Sun, so its depletion lets more ultraviolet radiation reach the Earth, which increases the risk of skin cancer. A: this describes the greenhouse effect, which is not the cause of ozone depletion. B: CFCs do not remove oxygen from the air and do not stop ozone forming; they destroy ozone that is already there. C: acid rain is caused by sulfur dioxide and nitrogen oxides, not by CFCs.