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

A: sodium chloride dissolves in water and sand does not. Adding water and stirring dissolves the salt; filtering leaves the sand on the filter paper as the residue, and the filtrate is a solution of sodium chloride. Heating the filtrate until it is saturated (crystals start to form) and leaving it to cool gives crystals of sodium chloride. B: the sand is still mixed in with the solution, so the crystals would form together with the sand and filtering would not separate them. C: distillation collects the water; the sodium chloride and the sand stay behind together, so filtering the residue separates nothing. D: the sand is the residue and it contains no sodium chloride; the dissolved sodium chloride is in the filtrate.

D: the protons and neutrons (nucleons) are in the nucleus. The number of neutrons is the nucleon number minus the proton number: 28 − 14 = 14. The 14 electrons are arranged in shells: 2 in the first shell, 8 in the second and 4 in the third, so there are three shells (2.8.4). A: 28 is the nucleon number, the total of protons and neutrons, so there are 14 neutrons, not 28. B: the first two shells hold only 2 + 8 = 10 electrons, so 14 electrons need three shells. C: neutrons are nucleons and are in the nucleus with the protons, not in the shells.

B: a 3+ ion has lost three electrons, so the atom had 28 + 3 = 31 electrons and therefore 31 protons. The heavier isotope has nucleon number 71, so the number of neutrons is 71 − 31 = 40. A: 38 is the number of neutrons in the lighter isotope (69 − 31). C: 43 comes from using the 28 electrons of the ion as the number of protons (71 − 28). D: 46 comes from taking the three electrons away from 28 instead of adding them back (71 − 25).

B: the oppositely charged ions in the lattice attract each other strongly, so the solid is hard. A hard blow moves a layer of ions along by one position, so ions with the same charge come next to each other and repel; the lattice splits apart and the solid is brittle. A: this describes a metal, whose layers of atoms can slide over each other without breaking; the forces between the layers of ions in an ionic compound are strong. C: an ionic lattice is held together by the attraction between positive and negative ions; its electrons are not free to move. D: magnesium oxide is not made of molecules; it is a giant lattice of ions.

A: statements 1, 2 and 3 are correct. Each Si–H bond is a shared pair of electrons, so there are four shared pairs. The silicon atom has its own 4 outer electrons plus 4 shared from the hydrogen atoms, which makes 8 (the noble gas arrangement of argon). Each hydrogen atom has its own electron plus 1 shared from silicon, which makes 2 (the noble gas arrangement of helium). Statement 4 is not correct: in covalent bonding electrons are shared, not transferred; transfer of electrons happens in ionic bonding.

C: the equation shows 1 Cu forming 2 FeCl₂, so two moles of iron(II) chloride form for each mole of copper that reacts. The state symbol (aq) means the iron(II) chloride is dissolved in water. A: the symbol (s) means the copper is a solid, not a solution. B: the coefficient of FeCl₃ is 2, so one mole of copper reacts with two moles of iron(III) chloride. D: the symbol (aq) after CuCl₂ means copper(II) chloride is dissolved in water; a solid would be shown by (s).

C: M_r of CaCO₃ = 40 + 12 + (3 × 16) = 100, so 100 g is 100 ÷ 100 = 1 mol. The equation shows that 1 mol of CaCO₃ gives 1 mol of CaO, so 1 mol of CaO forms. Mass of CaO = 1 × 56 = 56 g. A: 28 g is half of the correct mass; the equation gives 1 mol of CaO for every 1 mol of CaCO₃, not 0.5 mol. B: 44 g is the mass of the other product, carbon dioxide (1 × 44). D: 100 g is the mass of calcium carbonate that was heated; the carbon dioxide escapes, so the solid left weighs less.

D: an alkali produces hydroxide ions, OH⁻, in aqueous solution, and Universal Indicator turns blue or purple in an alkaline solution. A, B: hydrogen ions, H⁺, are produced by an acid, not an alkali. A, C: red or orange is the colour of Universal Indicator in an acidic solution.

A: a base reacts with an ammonium salt on warming and drives out ammonia gas (NH₄Cl + NaOH → NaCl + NH₃ + H₂O). Ammonia is identified because it turns damp red litmus paper blue. B: carbon dioxide is given off when an acid reacts with a carbonate; there is no carbonate here. C: chlorine is not formed; the chloride ions stay in solution as sodium chloride. D: hydrogen is given off when an acid reacts with a reactive metal, not when a base reacts with an ammonium salt.

C: iron(III) ions, Fe³⁺, give a red-brown precipitate of iron(III) hydroxide with aqueous sodium hydroxide, and it is insoluble in excess. A: copper(II) hydroxide is a light blue precipitate. B: iron(II) hydroxide is a green precipitate. D: zinc hydroxide is a white precipitate that dissolves in excess sodium hydroxide to give a colourless solution.

B: the colourless potassium iodide turns brown because iodine forms, so Y has oxidised the iodide ions: Y is an oxidising agent. The purple colour of acidified potassium manganate(VII) does not change, so Y has not reduced it: Y is not a reducing agent. A: a reducing agent would turn the purple acidified manganate(VII) colourless. C: Y would also have to decolourise the manganate(VII); it did not. D: the brown colour with potassium iodide shows that Y is an oxidising agent.

A: metallic character decreases from left to right across a period, so non-metallic character increases: sodium (on the left) is a metal and chlorine (on the right) is a non-metal. B: non-metallic character does not stay the same; it changes from metals on the left to non-metals on the right. C: this shows non-metallic character falling, which is the trend for metallic character; the graph has the wrong quantity on its axis. D: non-metallic character does not rise and then fall; it rises steadily from the metals on the left to the non-metals on the right.

D: chlorine is more reactive than bromine, so it displaces bromine from potassium bromide: Cl₂ + 2KBr → 2KCl + Br₂. The bromine that forms turns the colourless solution red-brown. A, B: the solution does change colour, because a reaction takes place. A, C: chlorine is above bromine in Group 17, so it is more reactive; a less reactive halogen cannot displace a more reactive one.

C: a reaction shows the added metal is more reactive than the metal in solution. W reacts with Z nitrate, so W is more reactive than Z. Y reacts with W nitrate, so Y is more reactive than W. Z reacts with X nitrate, so Z is more reactive than X. Putting the three results together gives Y, then W, then Z, then X. The two results with no reaction agree: Z does not react with Y nitrate and X does not react with W nitrate. A: this puts W above Y, but Y displaces W. B: this is the order of increasing reactivity, with the least reactive metal first. D: this puts Z above W, but W displaces Z.

A: the temperature of the mixture with W falls (29.0 → 25.5 °C, a drop of 3.5 °C), so energy is taken in from the surroundings: the change is endothermic. B: X has the lowest final temperature, but its temperature rises from 19.0 to 23.0 °C, so energy is given out and the change is exothermic. C: when the temperature does not change there is no net transfer of energy to or from the surroundings; the change is neither exothermic nor endothermic. D: the temperature of the mixture with Z rises from 27.0 to 31.0 °C, so energy is given out and the change is exothermic; the size of the final temperature does not matter, only the change.

C: statements 2 and 3 are correct. Natural gas and crude oil are non-renewable because they take millions of years to form and are used up much faster than that, so the amount in the ground is fixed; with no new reserves, using the gas twice as fast makes it run out in half the time. Statement 1 is not correct: bioethanol is a renewable fuel because the sugarcane it is made from can be grown again (the carbon dioxide taken in by the growing cane offsets the carbon dioxide given out when the fuel burns). Statement 4 is not correct: crude oil is non-renewable because it is used faster than it can be replaced, not because it forms carbon dioxide on burning (bioethanol also forms carbon dioxide when it burns). A, B and D each contain a statement that is not correct.

D: balancing the atoms shows that X must be HCl: one hydrogen atom of methane is replaced by a chlorine atom, and that hydrogen atom joins the other chlorine atom to form hydrogen chloride. Methane is an alkane (saturated), so it reacts with chlorine in ultraviolet light by substitution, one atom replacing another. A, B: the formula would be wrong: CH₄ + Cl₂ → CH₃Cl + H₂ does not balance (2 Cl on the left, 1 on the right). A, C: addition reactions need a C=C double bond, as in an alkene; methane has only single bonds.

B: polyunsaturated means that each molecule has more than one C=C double bond; this chain has two. A: this chain has only single bonds, so it is part of a saturated fat. C, D: each of these chains has just one C=C double bond, so it is monounsaturated, not polyunsaturated.

A: X contains the –CO₂H group, so it is a carboxylic acid (propanoic acid). Alcohols are oxidised to carboxylic acids, so X is formed when propanol is oxidised. B: ethanol is an alcohol, which contains the –OH group; X contains –CO₂H, so it is in the carboxylic acid series. C: a hydrocarbon contains only carbon and hydrogen; X also contains oxygen. D: X has no C=C double bond (the C=O double bond does not react with aqueous bromine), so it does not decolourise aqueous bromine.

B: melting the waste into pellets is a physical method, because the poly(ethene) is changed only in shape and no new substance forms. Cracking breaks the long polymer molecules into smaller molecules that can be used as fuel, so it is a chemical method. A: melting into pellets forms no new substance, so method 1 is not chemical. C: this has the two methods the wrong way round. D: cracking forms new substances, so method 2 is not physical.