CHEMISTRY – Paper 1 Multiple Choice – mark scheme

B: a burette has a tap, so the alkali can be run in a drop at a time, and its scale shows the volume that has been added. A: a measuring cylinder has no tap and its scale is not accurate enough for a titration. C: a pipette delivers one fixed volume only; it cannot add alkali a drop at a time and has no scale to read the volume added. D: a gas syringe is used to measure the volume of a gas, not of a liquid.

D: the fat and the juices are liquids that do not mix, so they form two layers; the lower layer can be run off through the tap of a separating funnel. A: crystallisation obtains crystals of a dissolved solid from its solution; neither liquid here is a dissolved solid. B: filtration separates an insoluble solid from a liquid; both the fat and the juices are liquids, so both would pass through the filter paper. C: sublimation separates a solid that changes directly into a gas from other solids; neither of these liquids does that.

D: the dye in the drink has moved 8.5 cm and the solvent front has moved 10.0 cm. R_f = distance moved by the dye ÷ distance moved by the solvent = 8.5 ÷ 10.0 = 0.85, the value for colouring M. A: J has an R_f value of 0.15 = (10.0 − 8.5) ÷ 10.0; the distance has been measured from the solvent front instead of from the baseline. B, C: the R_f values of K and L (0.35 and 0.60) are not the value for this spot.

C: X and Z both melt at exactly −114 °C and boil at exactly 78 °C; sharp, identical values show that they are the same pure substance (ethanol). A: W and X have different melting and boiling points, so they are different substances. B: W is pure water (0 °C and 100 °C), but Y melts lower and boils over a range of temperatures, so it is impure water with something dissolved in it; it is not the same pure substance. D: Y and Z have different melting and boiling points, and Y is a mixture.

A: the colouring spreads through still water (a liquid) and the smell spreads through still air (a gas) only because the particles of the liquid and of the gas are moving randomly and mixing (diffusion). B: particles held in fixed positions could not spread out and mix. C, D: each observation shows movement, one in a liquid and the other in a gas, so the particles of both liquids and gases move.

C: at a higher temperature the particles have more kinetic energy, so they move faster and spread out faster. A: the opposite is true; a gas with a lower relative molecular mass has lighter particles, which move faster, so it diffuses faster. B: a lower temperature gives the particles less kinetic energy, so they move more slowly and diffuse more slowly; the spacing of the particles does not change the rate in this way. D: at the same temperature the lighter particles move faster, so gases with different relative molecular masses diffuse at different rates.

D: a magnesium atom (2,8,2) loses its two outer electrons to form Mg²⁺, which has a full outer shell of 8 electrons; each of two chlorine atoms (2,8,7) gains one electron to form Cl⁻, also with 8 outer electrons. The ratio is one Mg²⁺ to two Cl⁻, so the charges balance. A: the magnesium ion is drawn still holding its own two outer electrons, although its 2+ charge shows that it has lost them. B: the chloride ions are drawn with only seven outer electrons; the electron gained from magnesium is missing, so the ions could not have a 1− charge. C: only one chloride ion is drawn, which is the formula MgCl; the charges (2+ and 1−) do not balance.

A: sugar is made of carbon, hydrogen and oxygen atoms chemically bonded together in a fixed ratio, so it is a compound. B: water is a compound, because each molecule contains atoms of two different elements; all the molecules being the same shows only that it is pure. C: salt solution is a mixture; the salt and the water are not chemically bonded and the proportions can vary. D: baking soda (sodium hydrogencarbonate) is a compound; a compound can contain any number of elements, as long as they are chemically combined in a fixed ratio.

D: the 2 outside the bracket multiplies every atom inside it: nitrogen 1 × 2 = 2 and hydrogen 4 × 2 = 8. Adding the 1 carbon atom and the 3 oxygen atoms gives 2 + 8 + 1 + 3 = 14 atoms. A: the 2 outside the bracket has been ignored (1 + 4 + 1 + 3 = 9). B: the 2 has been applied to the nitrogen only (2 + 4 + 1 + 3 = 10). C: the 2 has been applied to the hydrogen only (1 + 8 + 1 + 3 = 13).

C: n(CO₂) = 0.24 ÷ 24 = 0.010 mol. The equation shows 2 mol of NaHCO₃ for 1 mol of CO₂, so n(NaHCO₃) = 0.020 mol and its mass = 0.020 × 84 = 1.68 g. Percentage purity = 1.68 ÷ 4.2 × 100 = 40. A: the ratio is used the wrong way round (n(NaHCO₃) = 0.0050 mol, 0.42 g). B: the 2 : 1 ratio is ignored (n(NaHCO₃) = 0.010 mol, 0.84 g). D: this is the percentage of impurities (100 − 40) instead of the percentage of baking soda.

B: neutralisation is the reaction of hydrogen ions, from an acid, with hydroxide ions, from an alkali, to form water. A: this is the precipitation of silver chloride from silver ions and chloride ions. C: this is the reaction of a metal with an acid; magnesium displaces hydrogen. D: this is a displacement reaction in which zinc displaces copper from aqueous copper(II) ions.

A: 1 and 2. A base reacts with an acid (dilute sulfuric acid) to form a salt and water, and with an ammonium salt (ammonium nitrate) to give off ammonia gas, as well as a salt and water. 3, 4: sodium nitrate and potassium sulfate are salts that contain no ammonium ions and are not acids, so neither reacts with aqueous sodium hydroxide. B, C, D each include 3 or 4.

B: magnesium carbonate is insoluble in water, so an excess of it is added to the acid until no more reacts, and the unreacted solid is removed by filtration. Sodium hydroxide is soluble, so an excess could not be filtered off; instead the exact volumes of alkali and acid that just neutralise each other are found by titration with an indicator, and the reaction is then repeated without the indicator. A, C: precipitation is used to make an insoluble salt, but sodium sulfate is soluble (C is also wrong for magnesium sulfate). D: the two methods have been swapped.

B: the nitrogen is obtained from the air (about four-fifths of the air is nitrogen) and the hydrogen is obtained from the cracking of hydrocarbons from crude oil. A, D: the air does not contain useful amounts of hydrogen (A), and cracking hydrocarbons gives hydrogen, not nitrogen, because hydrocarbons contain no nitrogen (D). C: the sources are the wrong way round.

C: aluminium ions give a white precipitate with both reagents. The precipitate dissolves in excess aqueous sodium hydroxide, to give a colourless solution, but not in excess aqueous ammonia. A: these are the observations for iron(II) ions (a green precipitate with both reagents). B: these are the observations for calcium ions (there is no precipitate with aqueous ammonia). D: these are the observations for zinc ions, whose precipitate dissolves in excess of both reagents.

D: calcium carbonate and an acid give carbon dioxide, which turns limewater milky (it forms a white precipitate). A: magnesium and an acid give hydrogen, which ‘pops’ with a lighted splint; oxygen is the gas that relights a glowing splint. B: an ammonium salt and an alkali give ammonia, which turns damp red litmus paper blue; chlorine is the gas that bleaches damp litmus paper. C: zinc and an acid give hydrogen, which ‘pops’ with a lighted splint; the litmus result is the test for ammonia.

A: an oxidising agent oxidises colourless iodide ions to brown iodine, so P is an oxidising agent. A reducing agent reduces purple manganate(VII) ions to almost colourless manganese(II) ions, so Q is a reducing agent. B: the roles of P and Q are the wrong way round. C: Q cannot be an oxidising agent; acidified potassium manganate(VII) is itself an oxidising agent and is decolourised only by a reducing agent. D: P cannot be a reducing agent; a reducing agent does not turn aqueous potassium iodide brown.

B: statements 1 and 3. Statement 1 is correct: a compound is decomposed at the electrodes only if it contains charged particles, ions, that move to them. Statement 3 is correct: in the solid the ions are held in fixed positions in a lattice, so they cannot move and the solid does not conduct. Statement 2 is wrong: in the molten compound the charge is carried by the moving ions; electrons move in the wires and the electrodes, not through the electrolyte.

C: molten potassium chloride contains only potassium ions and chloride ions. Potassium ions gain electrons at the negative electrode to form potassium, and chloride ions lose electrons at the positive electrode to form chlorine. A, B: hydrogen is formed from the water in an aqueous solution; there is no water in the molten compound. B, D: oxygen comes from the discharge of hydroxide ions in an aqueous solution; there are none in the molten compound.

D: the elements are arranged in order of increasing proton number. Potassium (19 protons) has one more proton than argon (18 protons), so it comes after argon, even though its nucleon number is smaller. A: argon has the larger nucleon number (40 compared with 39); the Periodic Table is not arranged in order of nucleon number. B: an argon atom has 40 − 18 = 22 neutrons and a potassium atom has 39 − 19 = 20 neutrons, so argon has more neutrons, not fewer; the number of neutrons does not decide the order either. C: argon (2,8,8) has three electron shells and potassium (2,8,8,1) has four, so argon has fewer shells, not more.

C: chlorine and iodine are both in Group 17, so an atom of each has seven electrons in its outer shell (the same number). It is the outer shell electrons that decide how an element reacts, so elements in the same group react in a similar way (each atom gains one electron to form a 1⁻ ion). The number of electron shells is different (chlorine has 3, iodine has 5) because the atoms get larger down a group. A, B: atoms with different numbers of outer shell electrons would not have similar chemical properties. B, D: the numbers of electron shells are not the same; atoms of elements in the same period, not the same group, have the same number of shells.

D: an atom with 6 electrons in its outer shell (sulfur) is an atom of a non-metal: atoms of non-metals have four or more outer shell electrons, which they gain or share when they react. A, B, C: atoms with 1, 2 or 3 outer shell electrons (sodium, magnesium and aluminium) are atoms of metals, which lose their outer shell electrons to form positive ions. All four atoms have three electron shells, so it is the number of outer shell electrons that decides the type of element.

C: nickel is a transition element, and transition elements and their compounds are often able to act as catalysts (iron, nickel and platinum are used as catalysts in industry). A: argon is a noble gas; its atoms are unreactive and take no part in reactions. B: chlorine is a reactive non-metal (a halogen) that is used up in its reactions, so it is not a catalyst. D: sodium is a Group 1 metal, not a transition element; it reacts vigorously and is used up, so it is not a catalyst.

B: a metal displaces the ions of a metal that forms positive ions less readily than it does. R forms positive ions less readily than zinc, so R cannot displace zinc from aqueous zinc sulfate (no reaction). R forms positive ions more readily than iron, so R atoms lose electrons to Fe²⁺ ions and iron is displaced from aqueous iron(II) sulfate (reaction). A: no reaction with aqueous iron(II) sulfate would mean that R forms positive ions less readily than iron does. C: this is the wrong way round; a metal that could displace zinc ions would also displace iron ions, because zinc forms positive ions more readily than iron does. D: R would have to form positive ions more readily than zinc does, which it does not.

A: the more reactive the metal, the more stable its carbonate is to heat. Magnesium is higher in the reactivity series than zinc, so magnesium carbonate needs the higher temperature to decompose. B: magnesium carbonate is the right carbonate, but magnesium is higher, not lower, in the reactivity series than zinc. C: zinc is lower, not higher, in the reactivity series than magnesium, so zinc carbonate is less stable to heat. D: zinc is lower in the reactivity series than magnesium, but this makes zinc carbonate decompose at the lower temperature, not the higher one.

A: statements 1 and 2 are correct. A layer of oil, or of zinc (galvanising), is a barrier that keeps oxygen and water away from the iron, so the iron cannot rust. Statements 3 and 4 are wrong: iron rusts only when both oxygen and water are present. In dry air there is oxygen but no water (statement 3); boiled water contains no dissolved oxygen and the layer of oil keeps air out, so there is water but no oxygen (statement 4). B: statement 3 is wrong. C: statement 4 is wrong. D: both statements are wrong.

D: in an exothermic reaction more energy is released when the bonds in the products form than is absorbed when the bonds in the reactants break, so the overall energy change is negative and heat passes to the surroundings; only statement 3 is correct. Statements 1 and 2 are the wrong way round: breaking bonds absorbs energy and making bonds releases energy. A: both statements are wrong. B: statement 1 is wrong. C: statement 2 is wrong.

B: experiments 1 and 3 differ only in the size of the pieces of eggshell (large and small). The mass of eggshell, the volume of vinegar and the temperature are the same, so any difference in the volume of gas is due to the size of the pieces. A: experiment 2 changes the size and the temperature, so the effect of size cannot be separated from the effect of temperature. C: experiment 4 changes the size and the mass of eggshell. D: experiment 5 changes the volume of vinegar, not the size of the pieces.

A: all three statements are correct. The members of a homologous series can be represented by one general formula (for example C_nH_2n+2 for the alkanes), they have similar chemical properties because they have the same functional group, and physical properties such as boiling point show a gradation (they change steadily) as the molecules get larger. B: leaves out statement 3. C: leaves out statement 1. D: leaves out statements 1 and 3.

C: alkenes have the general formula C_nH_2n, so 2n = 14 and n = 7. The formula is C₇H₁₄ and M_r = 7 × 12 + 14 × 1 = 98. A: 84 = 7 × 12 is the mass of the carbon atoms only; the hydrogen atoms have been left out. B: 86 is the M_r of C₆H₁₄, an alkane (general formula C_nH_2n+2), not an alkene. D: 110 is the M_r of C₈H₁₄, which has 14 hydrogen atoms but too many carbon atoms for an alkene (it would need 16).

D: polyunsaturated molecules contain more than one carbon–carbon double bond; the molecules of S contain three. A, C: P and R each have one C=C bond, so they are unsaturated but not polyunsaturated. B: Q has no C=C bonds (only single bonds between carbon atoms), so it is saturated.

B: propan-1-ol contains the –OH group joined to a carbon atom that is not part of a C=O group; the –OH group is the functional group of the alcohols. A: ethanoic acid also has an O–H bond, but it is part of the –CO₂H group of a carboxylic acid. C: propene is a hydrocarbon (an alkene) containing a C=C double bond; it has no oxygen. D: methyl ethanoate is an ester; its –CO₂– group has no –OH.

A: CH₃CH₂CO₂H (propanoic acid) contains the –CO₂H group, the functional group of the carboxylic acids. B: CH₃CH₂CH=CH₂ (but-1-ene) is an alkene; it has a C=C double bond and no oxygen. C: CH₃CH₂CH₂CH₂OH (butan-1-ol) is an alcohol; it has an –OH group. D: CH₃CO₂CH₂CH₃ (ethyl ethanoate) is an ester: its –CO₂– group is joined to carbon atoms on both sides, not to H, so it has no –CO₂H group.

C: in both changes the ethanol gains oxygen (CH₃CH₂OH becomes CH₃CO₂H), so it is oxidised. Acidified manganate(VII) ions and atmospheric oxygen are both oxidising agents; it is the manganate(VII) that is reduced, which is why the purple colour fades. A: gaining oxygen is oxidation, not reduction. B, D: both manganate(VII) ions and oxygen are oxidising agents, so the ethanol is changed in the same way by each; neither can reduce it.

A: the name of an ester starts with the alkyl group of the alcohol (ethyl) and ends with the name of the acid (methanoate): ethyl methanoate. One molecule of water is lost when the acid and the alcohol join, so CH₂O₂ + C₂H₆O − H₂O = C₃H₆O₂. B, D: C₃H₈O₃ is the sum of the formulae of the two reactants; the water that is also formed has been forgotten. C, D: the name is the wrong way round; methyl ethanoate is made from ethanoic acid and methanol (it has the same formula as ethyl methanoate, C₃H₆O₂).

B: in addition polymerisation the C=C double bond of each ethene molecule opens up and the molecules join into a long chain of single-bonded carbon atoms, each carbon carrying two hydrogen atoms: –CH₂–CH₂–. A: this chain still has C=C double bonds; the polymer of an alkene is saturated because the double bonds are used up in joining the monomers. C: this is poly(propene), made from propene, CH₃CH=CH₂. D: this is poly(chloroethene), made from chloroethene, CH₂=CHCl.

D: the units are joined by –CO–O– (ester) links, so the polymer is a polyester, Terylene. It is a condensation polymer: a small molecule (water) is lost each time a link forms. A: nylon is a polyamide; its units are joined by –CO–NH– links. B, C: poly(ethene) and poly(propene) are addition polymers made from alkenes; the chain is a backbone of carbon atoms with no –CO–O– links.

D: nylon and Terylene are made into fibres for clothing, curtain materials and sleeping bags (as well as fishing line and parachutes). Plastic carrier bags and clingfilm are made from poly(ethene), a plastic, not from a fibre. A, B, C: each includes item 1 or item 2, or both.

B: when carbon-containing substances such as charcoal burn in a limited supply of air, the combustion is incomplete and carbon monoxide is formed. A: carbon dioxide is the product of complete combustion, in a plentiful supply of air. C: nitrogen oxides come from lightning and from internal combustion engines. D: sulfur dioxide comes from volcanoes and from burning fossil fuels that contain sulfur; charcoal is almost pure carbon.

C: chlorofluorocarbons (CFCs) release chlorine atoms that react with ozone and break it down. The ozone layer protects living things by absorbing harmful ultraviolet radiation from the Sun; when it is depleted, more ultraviolet radiation reaches the Earth, which increases the risk of skin cancer. A: carbon dioxide does not deplete the ozone layer; it is a greenhouse gas that contributes to global warming. B: neither part is correct; carbon dioxide does not deplete the ozone layer, and the radiation that the ozone layer absorbs is ultraviolet, not infrared. D: CFCs do deplete the ozone layer, but the radiation that the ozone layer absorbs is ultraviolet, not infrared.