CHEMISTRY – Paper 1 Multiple Choice – mark scheme

A: concentrated sulfuric acid is an acid, so it does not react with the acidic gas hydrogen chloride and can dry it; hydrogen chloride is denser than air, so it is collected by downward delivery in an upright jar, and the air is pushed out of the top. B: calcium oxide is a basic oxide and reacts with an acidic gas such as hydrogen chloride, so it cannot be used to dry it. C: collecting in an upside-down jar (upward delivery) is only for gases that are less dense than air; hydrogen chloride is denser than air, so it would fall out of the open jar. D: hydrogen chloride is very soluble in water, so it cannot be collected over water, and a gas collected over water is wet, not dry.

B: iodine sublimes: it changes straight from solid to gas on gentle heating, and the gas turns back to solid iodine on a cold surface, leaving the sand behind. A: iodine is almost insoluble in water and sand is insoluble, so both stay in the residue and are not separated. C: evaporating the water leaves both solids in the dish. D: strong heating in an open dish drives the iodine off as a gas into the air, so the solid that is left is the sand, not iodine.

D: sugars are colourless, so their spots cannot be seen on the paper; the locating agent reacts with them to give coloured spots (statement 2), which is why it is needed. Statement 1 is wrong: the sugars dissolve in the solvent, which carries them up the paper before the locating agent is applied; the locating agent does not make them dissolve or move. Statement 3 is wrong: the sugars are separated by the solvent as it rises up the paper, because different sugars travel different distances; the locating agent is sprayed on afterwards and only makes the separated spots visible. A includes statements 1 and 3, B is statement 1 only, and C includes statement 3.

A: to change from liquid to gas, sweat particles must overcome the attractive forces holding them together, so they take in energy from the skin and move far apart; the skin loses energy and feels cooler. B: taking in energy is right, but particles move further apart (not closer together) when a liquid becomes a gas. C: giving out energy to the skin would warm it; this is the energy change when a gas condenses. D: giving out energy and moving closer together is the change when water vapour condenses, the reverse of evaporation.

B: the smell is caused by gas particles that leave the fruit; they move randomly in all directions and spread out among the particles of the air (diffusion), from where there are many of them to where there are few. A: the particles do not change size; they spread out because they move. C: the particles move in all directions, not only upwards; the smell is detected sideways and a few metres away. D: diffusion happens without wind, because the particles move on their own.

D: a covalent bond forms when two atoms share a pair of electrons: each atom provides one electron. The hydrogen atom (1 electron) then has 2, like helium, and the chlorine atom (7 outer electrons) then has 8, like argon, so each atom has the electronic configuration of a noble gas. A: counts only the outer electrons each atom had before bonding, so the shared pair is ignored. B: counts the shared pair for chlorine but not for hydrogen. C: counts the shared pair for hydrogen but not for chlorine.

A: the smaller copper atoms disturb the regular layers of gold atoms (see the diagram), so when a force is applied the layers cannot slide over each other as easily; the alloy is harder than pure gold. B: the arrangement in the alloy is less regular, not more regular, as the diagram shows. C: an alloy is a mixture of a metal with another element, not a compound; its atoms are held by metallic bonding. D: the delocalised electrons still move through the alloy (it still conducts electricity); the hardness comes from the layers being unable to slide.

C: ammonia is NH₃ (one nitrogen atom and three hydrogen atoms); calcium hydroxide has one Ca²⁺ ion and two OH⁻ ions, so the formula is Ca(OH)₂; carbon monoxide is CO (one carbon atom and one oxygen atom). A: CO₂ is the formula of carbon dioxide, which has two oxygen atoms. B: CaOH₂ is wrong because calcium hydroxide contains two hydroxide ions, so the OH must be in brackets: Ca(OH)₂. D: NH₄ is not the formula of a molecule; the ammonium ion is NH₄⁺.

D: the coefficients (all 1) show that one mole of calcium oxide reacts with one mole of water, and the state symbol (s) after Ca(OH)₂ shows that the calcium hydroxide formed is a solid. A: the water is shown as (l), a liquid; only the calcium oxide is a solid. B: a solution would be shown as (aq); the calcium hydroxide is shown as (s), a solid. C: there is no coefficient in front of H₂O, so one mole of water, not two, reacts with one mole of calcium oxide.

B: calcium carbonate (1) reacts with the acid to give carbon dioxide, and zinc (4) is above hydrogen in the reactivity series, so it reacts to give hydrogen. Magnesium oxide (3) is a base, so it reacts with the acid to give only a salt and water, with no gas. Copper (2) is below hydrogen in the reactivity series and does not react with dilute acid. A: magnesium oxide forms no gas. C: neither copper nor magnesium oxide forms a gas. D: copper does not react with dilute hydrochloric acid.

C: calcium hydroxide is a base (an alkali): its hydroxide ions neutralise the hydrogen ions in the soil, so the concentration of hydrogen ions decreases and the pH increases towards 7, which suits most crops. A: a lower concentration of hydrogen ions means a higher pH, so the pH and the concentration of hydrogen ions cannot both decrease. B: a pH that decreases with a concentration of hydrogen ions that increases is the effect of adding an acid, which makes the soil more acidic. D: a higher concentration of hydrogen ions means a lower pH, so the pH and the concentration of hydrogen ions cannot both increase.

D: aluminium oxide, the oxide of a metal, is amphoteric (it reacts with both acids and alkalis); carbon monoxide is a neutral oxide (a non-metal oxide that does not react with acids or alkalis); sulfur dioxide, the oxide of a non-metal, is acidic. A: carbon monoxide is not acidic; it does not react with alkalis. B: sulfur dioxide dissolves in water to give an acid and reacts with alkalis, so it is acidic, not neutral. C: aluminium oxide reacts with alkalis as well as acids, so it is amphoteric, not simply basic.

B: calcium carbonate is insoluble, so it forms as a precipitate when a solution containing calcium ions is mixed with a solution containing carbonate ions; calcium chloride and sodium carbonate are both soluble, and the other product, sodium chloride, stays in solution, so the calcium carbonate can be filtered off, washed and dried. A: a precipitate forms, but it is barium carbonate, not calcium carbonate, because there are no calcium ions. C: there are no carbonate ions; the products, calcium nitrate and sodium chloride, are both soluble, so there is no precipitate. D: there are no calcium ions; the products, potassium nitrate and sodium carbonate, are both soluble.

A: the manufacture of ammonia is a reversible reaction: ammonia breaks down to nitrogen and hydrogen as well as forming from them, so the mixture contains all three gases and does not become pure ammonia. B: if the reaction were only slow, the amounts would still be changing; they no longer change. C: nitrogen and hydrogen do react to form ammonia; the reaction goes in both directions, not in one only. D: the stem states that the gases were mixed in the exact ratio needed to make ammonia, so neither gas is in excess.

C: no change with dilute acid shows there is no carbonate; the white precipitate with barium nitrate in acid shows sulfate; the white precipitate with silver nitrate in acid shows chloride (silver chloride is white). A: a carbonate would give effervescence with dilute nitric acid. B: if sulfate were absent, there would be no precipitate with barium nitrate. D: silver iodide is yellow; a white precipitate with silver nitrate shows chloride, not iodide.

D: oxidation is gain of oxygen (or loss of hydrogen). In 2, iron gains oxygen to form iron(III) oxide; in 3, magnesium gains oxygen to form magnesium oxide: both are oxidised. In 1, nitrogen gains hydrogen to form ammonia, which is reduction (gain of hydrogen), not oxidation. A, B, C: each includes reaction 1, which is a reduction of nitrogen; B and C also leave out one of the two oxidations.

B: sea water contains ionic compounds (salts) dissolved in water, so it contains mobile ions; the ions carry the current through the sea water, and at the electrodes they are changed into new substances, which is why gas bubbles form at both electrodes. A: electrons carry the current in the wires, not through the solution; ions carry it in the electrolyte. C: electrolysis is the passing of electricity through a molten or dissolved ionic compound with chemical changes at the electrodes; the ions are changed there. D: sugar is a molecular compound whose solution contains no ions, so a solution of sugar does not conduct electricity and no electrolysis takes place.

A: in molten sodium chloride the Na⁺ and Cl⁻ ions are free to move; the negative chloride ions are attracted to the positive electrode, where each loses an electron to form chlorine. B: the ions attracted are right, but sodium is not formed at the positive electrode; it forms at the negative electrode. C, D: the positively charged sodium ions are attracted to the negative electrode, not the positive electrode (D also gives sodium as the product, which forms at the negative electrode).

D: in a hydrogen fuel cell, hydrogen reacts with oxygen to generate electricity directly; the only product is water, so no carbon dioxide is released. A: a fuel cell does not burn the fuel to make heat; the electricity is produced directly by the reaction. B: hydrogen contains no carbon, so no carbon dioxide forms; water is the only product. C: hydrogen can be obtained from water and also from hydrocarbons, for example by cracking.

C: X has 13 electrons, arranged 2,8,3: it loses its 3 outer electrons to form an ion with a charge of 3+. Y has 16 electrons, arranged 2,8,6: it gains 2 electrons to complete its outer shell of 8, forming an ion with a charge of 2−. A: the signs are reversed: the atom with 3 outer electrons loses them (a positive ion) and the atom with 6 outer electrons gains electrons (a negative ion). B: X is given a negative charge and Y a positive charge, and a charge of 6+ would need Y to lose 6 electrons; gaining 2 is easier. D: the charge on X is right, but the charge of 6− takes the number of outer electrons of Y as its charge; Y needs to gain only 2 electrons to reach 8.

D: across a period, from left to right, the elements change from metallic to non-metallic character. Sodium, on the left, is a metal and conducts electricity; sulfur, further right, is a non-metal and does not (statement 3). Chlorine is to the right of phosphorus, so it is the more non-metallic (statement 4). Statement 1 has the change the wrong way round, so A and B are wrong. Statement 2 is incorrect because magnesium is to the left of silicon and is therefore the more metallic, so A and C are wrong.

A: reactivity decreases down Group 17, so chlorine is more reactive than iodine and displaces iodine from the iodide ions: Cl₂ + 2I⁻ → 2Cl⁻ + I₂. The iodine formed makes the solution turn brown. B: the observation is right but the explanation is reversed; if chlorine were less reactive than iodine it could not displace it. C: if chlorine is more reactive than iodine a reaction must occur, so there would be a colour change. D: the explanation is wrong (chlorine is above iodine in Group 17), and so is the prediction of no change.

C: a metal that reacts with dilute hydrochloric acid is above hydrogen in the reactivity series, and a metal whose oxide carbon cannot reduce is above carbon. Magnesium is above both, so it fits both clues. A, D: copper and silver are below hydrogen, so neither reacts with dilute hydrochloric acid. B: iron reacts with the acid, but it is below carbon, so its oxide is reduced when heated with carbon.

C: a metal displaces a less reactive metal from a solution of its salt. Q displaces lead, so Q is more reactive than lead, that is above lead in the series. Q does not displace iron, so it is less reactive than iron, that is below iron. Q is therefore between iron and lead. A, B: a metal above iron in the series would be more reactive than iron, so it would displace iron from aqueous iron(II) sulfate. D: a metal below lead would be less reactive than lead and could not displace it.

C: calcium is high in the reactivity series, so its compounds are very hard to break down and it can only be extracted by electrolysis. Iron is lower in the series, below carbon, so its oxide can be reduced by heating it with carbon. A: iron is not extracted by electrolysis. B: both columns are the wrong way round. D: carbon cannot reduce calcium oxide, because calcium is above carbon in the reactivity series.

B: this is sacrificial protection. Metal M is more reactive than iron, so it has the greater tendency to lose electrons and form positive ions; it corrodes in preference to the steel, and the steel does not rust while M is present. A: a less reactive metal would not corrode in place of iron; the steel would corrode first. C: a less reactive metal could not protect the iron, and a block at one place cannot keep oxygen and water away from the whole leg. D: M is a block attached to one part of the leg; it does not cover the steel, so it is not acting as a barrier.

D: the rate at the start is shown by the gradient of the curve. The two curves start with the same gradient, so the rates at the start are the same; this is expected because the acid has the same concentration and the tablet is in excess in both experiments. A: curve Y finishes higher, but that is because there is more acid in experiment Y, not because the reaction is faster. B: curve X levels off earlier than curve Y, so experiment X stops first; experiment Y goes on longer because it has more hydrochloric acid to react. C: curve Y levels off at a higher volume, so more carbon dioxide forms in experiment Y.

C: natural gas is mainly methane. It is a fossil fuel that took millions of years to form and is being used far faster than it forms, so it is a non-renewable source of energy. A, B: butane is a main compound of liquefied petroleum gas (the gas sold in cylinders), not of natural gas. B, D: natural gas is not renewable, because new deposits do not form on a human timescale.

B: it has only single bonds between the atoms, so it is an alkane (C₄H₁₀), and its carbon chain has a branch: a CH₃ group is attached to the middle carbon atom of a three-carbon chain (2-methylpropane). A: butane is an alkane, but its four carbon atoms are in a straight chain, so it is unbranched. C: methylpropene has a branch, but it contains a C=C double bond, so it is an alkene. D: but-1-ene contains a C=C double bond, so it is an alkene, and its chain is unbranched.

A: isomers have the same molecular formula but different structural formulae. But-1-ene is C₄H₈. Structure 1 (methylpropene) and structure 2 (but-2-ene) are both C₄H₈ with a different arrangement of the atoms, so both are isomers of but-1-ene. Structure 3 is CH₃CH₂CH=CH₂, which is but-1-ene itself drawn the other way round: it is the same compound, not an isomer. Structure 4 is butane, C₄H₁₀, which has a different molecular formula. B, C, D each contain structure 3 or structure 4.

D: more of fractions R and S is obtained than is needed (38 against 30 and 42 against 20), and they contain the largest molecules. Cracking breaks large molecules into smaller, more useful ones (alkenes and hydrogen are also formed), so R and S are cracked to make more of the small molecules in fractions P and Q, which are in short supply (3 against 10 and 17 against 40). Cracking is essential to match the supply to the demand. A: P and Q are the fractions in short supply; they are made by cracking, not cracked. B, C: each contains a fraction (P or Q) of which too little is obtained, and cracking would only make the shortage worse.

B: margarine is made by adding hydrogen to unsaturated vegetable oils: hydrogen adds across the C=C double bonds in an addition reaction (statement 1), and the product is a solid, unlike the oil, which is a liquid (statement 2). Statement 3 is incorrect: the oil is unsaturated, because it has C=C double bonds, and that is why hydrogen can add to it. A, C and D each include statement 3.

C: alcohols burn completely to form carbon dioxide and water: 2CH₃OH + 3O₂ → 2CO₂ + 4H₂O. Amount of methanol = 3.2 ÷ 32 = 0.10 mol; each mole of methanol needs 1.5 mol of oxygen molecules, so 0.15 mol of O₂ is used, a mass of 0.15 × 32 = 4.8 g. A: 1.6 g is 0.05 mol of O₂ (half a mole for each mole of methanol). B: 3.2 g is 0.10 mol of O₂ (one mole for each mole of methanol, as if the equation were not balanced). D: 6.4 g is 0.20 mol of O₂ (two moles for each mole of methanol).

D: it contains the carboxylic acid group, –CO₂H, and three carbon atoms in total, two in the chain and the one in the –CO₂H group, so it is propanoic acid. A: this is ethanoic acid, which has two carbon atoms. B: this is butanoic acid, which has four carbon atoms. C: this has three carbon atoms but it contains the –OH group of an alcohol, not the –CO₂H group of a carboxylic acid; it is propan-1-ol.

B: a carboxylic acid reacts with an alcohol to form an ester and water, one molecule of each reactant giving one molecule of the ester and one molecule of water (the concentrated sulfuric acid is only a catalyst). Relative molecular mass of the ester = 60 + 60 − 18 = 102. A: 60 is the relative molecular mass of one reactant only. C: 120 = 60 + 60 adds the two reactants and forgets that a molecule of water is also formed. D: 138 = 120 + 18 adds the mass of the water instead of subtracting it.

A: poly(ethene) is a large molecule built up from many small ethene molecules (monomers) joined end to end. Each ethene molecule contributes two carbon atoms to the chain, so six carbon atoms come from 6 ÷ 2 = 3 ethene molecules. B: 5 is the number of C–C bonds between the six carbon atoms. C: 6 is the number of carbon atoms, but each ethene molecule contributes two of them. D: 12 is the number of hydrogen atoms in this part of the molecule.

C: poly(ethene) is a typical plastic that is easily moulded; plastic bags and clingfilm are two of its uses. A: clingfilm is made from poly(ethene), but a sleeping bag is made from synthetic fibres such as nylon or Terylene. B: a fishing line and a sleeping bag are both made from synthetic fibres (nylon and Terylene), not from poly(ethene). D: a plastic bag is made from poly(ethene), but a fishing line is made from a synthetic fibre such as nylon.

A: environmental issue: recycling plastics saves crude oil, a non-renewable resource, because less is needed as raw material (statement 1 is correct). Social and economic issue: collecting, sorting and processing plastic waste gives people jobs (statement 3 is correct). Statement 2 is incorrect: energy is needed to transport the waste and to melt or crack it, which adds to the cost and can add to pollution. Statement 4 is incorrect: plastics have to be sorted into their different types before they are recycled, because a mixture gives a poor-quality product. B, C and D each include statement 2 or statement 4.

A: dry air is about 78% nitrogen and 21% oxygen. The remaining 1% is mostly argon, the main noble gas in air, with only a trace of carbon dioxide. B: carbon dioxide is present at about 0.04%, far less than argon, even though it is often in the news because of global warming. C, D: helium and neon are noble gases, but they are present only in tiny traces.

B: in a catalytic converter nitrogen dioxide is changed into harmless nitrogen: it loses oxygen and the oxidation state of nitrogen decreases from +4 to 0, so the pollutant is reduced. A: carbon monoxide is oxidised to carbon dioxide (the oxidation state of carbon increases from +2 to +4). D: unburnt hydrocarbons are oxidised to carbon dioxide and water. C: sulfur dioxide is not removed by a catalytic converter; it is removed from the waste gases of power stations by calcium carbonate, or avoided by using low-sulfur fuel.