ESAT Chemistry: Bonding, structure and the groups, set 3

• Answer all questions. No calculator.
• Each question has exactly one correct answer.

1. 11 mark

   Ammonium ions carry a 1+ charge, and the phosphate ion has the formula PO4 with a 3- charge. What is the correct formula for ammonium phosphate, formed when these two ions combine?

   1. A NH4PO4
   2. B (NH4)3PO4
   3. C NH4(PO4)3
   4. D NH12PO4
2. 21 mark

   Ice melts, then the resulting water boils, and if heated further still the O-H covalent bonds within each water molecule could in principle be broken apart, atomising the molecule into separate hydrogen and oxygen atoms. Which lists these three processes correctly in order of increasing energy required, from least to most?

   1. A Melting the ice, then boiling the water, then breaking the O-H covalent bonds
   2. B Breaking the O-H covalent bonds, then melting the ice, then boiling the water
   3. C Melting the ice, then breaking the O-H covalent bonds, then boiling the water
   4. D Boiling the water, then melting the ice, then breaking the O-H covalent bonds
3. 31 mark

   Graphite is soft and slippery, used as a lubricant and in pencil leads, while diamond is the hardest known natural substance. Both are giant covalent structures made entirely of carbon atoms. Which statement correctly explains this difference in hardness?

   1. A Diamond is harder because its carbon atoms are heavier than the carbon atoms in graphite, and this greater atomic mass alone accounts for how strongly the atoms resist being forced apart.
   2. B Graphite is soft because its individual covalent bonds are weaker than the covalent bonds in diamond, so applying even a small force is enough to snap the bonds holding each layer's carbon atoms together.
   3. C In diamond, every carbon atom forms four strong covalent bonds in a rigid three-dimensional network; in graphite, strongly bonded flat layers are held together only by weak forces, letting them slide.
   4. D Diamond is harder because it contains additional impurity atoms locked into its lattice, which reinforce the structure and prevent the flat layers found in graphite from being able to form at all.
4. 41 mark

   Sodium has a melting point of about 98 degrees C, while magnesium, in the same period, has a melting point of about 650 degrees C. Which explanation best accounts for this difference?

   1. A Magnesium atoms are heavier than sodium atoms, and this greater atomic mass alone, independent of any difference in bonding, explains the much higher melting point.
   2. B Magnesium ions carry a greater charge (2+) than sodium (1+) and contribute two delocalised electrons rather than one, giving stronger electrostatic attraction and so a higher melting point.
   3. C Sodium atoms are held together by weak covalent bonds within molecules, while magnesium atoms are held together by strong ionic bonds to a non-metal, which explains magnesium's much higher melting point.
   4. D Magnesium has a higher melting point because its atoms are arranged in a giant covalent lattice like diamond, unlike sodium's simple metallic structure of ions and delocalised electrons.
5. 51 mark

   The melting points of the Group 1 metals decrease going down the group: lithium about 180 degrees C, sodium about 98 degrees C, potassium about 63 degrees C. Which explanation best accounts for this trend?

   1. A Going down the group, the metal atoms gain extra outer electrons, which strengthens the covalent bonds between neighbouring atoms in the lattice and so lowers the melting point.
   2. B Going down the group, the atoms become smaller because extra protons in the nucleus pull the outer electrons in more tightly, and this weakens the metallic bonding and lowers the melting point.
   3. C The melting point decreases because the metals become more reactive going down the group, and a more reactive metal always has a lower melting point regardless of its metallic bonding.
   4. D Going down the group, the metal ions become larger, so the delocalised electrons sit further from the ions' positive charge; this weakens the electrostatic attraction and lowers the melting point.
6. 61 mark

   Separate aqueous solutions of potassium chloride, potassium bromide and potassium iodide are set up. A student adds bromine water to a sample of each solution in turn. In which solution(s), if any, will a displacement reaction occur, and why?

   1. A In all three solutions, because bromine is a halogen and any halogen can displace the halide ion of any other halogen from solution, regardless of their relative reactivity.
   2. B In none of the solutions, because a displacement reaction can only occur between a halogen and a metal, and never between two non-metal halogens such as bromine and chloride or iodide.
   3. C Only in the potassium iodide solution: bromine is more reactive than iodine but less reactive than chlorine, so it displaces iodide ions but cannot displace chloride ions.
   4. D Only in the potassium chloride solution, because bromine is a heavier halogen than chlorine, and a heavier halogen always displaces a lighter halogen from its compounds.
7. 71 mark

   A mixture contains sand (insoluble in water), copper sulfate (soluble in water, forming blue crystals when its solution is concentrated) and ethanol (miscible with water, boiling point 78 degrees C, compared with water's 100 degrees C), all stirred together into water. Which sequence of steps correctly obtains all three components in a pure, separate form?

   1. A Filter the mixture to remove the sand; fractionally distil the filtrate to collect the ethanol; then crystallise the remaining copper sulfate solution to obtain crystals.
   2. B Fractionally distil the whole mixture first to collect the ethanol along with some of the water; then filter out the copper sulfate crystals; then wash the sand with water to remove any remaining ethanol.
   3. C Filter the mixture to remove the sand; use a separating funnel to separate the ethanol from the water as two immiscible layers; then evaporate the remaining water to leave copper sulfate.
   4. D Crystallise the whole mixture first while it still contains sand to obtain copper sulfate crystals; then filter to remove the sand; then fractionally distil the remaining liquid to obtain ethanol.
8. 81 mark

   A forensic chemist runs chromatography on an ink sample suspected of containing a particular dye, alongside a pure reference sample of that dye, using the same solvent, paper and temperature for both. The ink sample separates into three spots; one of these has the same Rf value as the reference dye's single spot. What can correctly be concluded?

   1. A The ink is a pure substance, because chromatography always separates a mixture into exactly three components.
   2. B The ink cannot contain the reference dye, because a pure substance always produces only one spot, and the ink produced three.
   3. C No conclusion can be drawn, because Rf values only compare distances travelled and never indicate what a substance is made of.
   4. D The ink is a mixture that contains the reference dye as one of its components, together with at least two other substances.
9. 91 mark

   Which pair of elements, when they react together, would be expected to form covalent bonds rather than ionic bonds?

   1. A Calcium and chlorine, because calcium loses two electrons to form Ca2+ and each chlorine atom gains one electron to form Cl-.
   2. B Carbon and chlorine, because both are non-metals and so share electrons rather than transferring them completely.
   3. C Potassium and oxygen, because potassium loses one electron and oxygen gains two, so their charges do not balance in a simple one-to-one ratio.
   4. D Magnesium and oxygen, because both elements are in Period 3, and Period 3 elements always bond covalently with each other.
10. 101 mark

    The halogens (Group 17) become less reactive going down the group, from fluorine to iodine. Which explanation correctly accounts for this trend?

    1. A Going down the group, the halogen molecules become heavier, and heavier molecules are always less reactive than lighter ones because their extra mass makes them harder to break apart during a reaction.
    2. B Going down the group, the number of outer-shell electrons increases, so each atom needs to gain more electrons than fluorine does to complete its outer shell, making reaction progressively less likely.
    3. C Going down the group, atoms become larger with an extra electron shell, so the nucleus attracts an incoming electron less strongly and shields it more, making it harder to react.
    4. D Going down the group, the covalent bond within each diatomic molecule becomes weaker, and this weaker internal bond directly makes the whole molecule less reactive towards other substances.
11. 111 mark

    A puddle of water disappears on a warm day, even though the air temperature is well below water's boiling point of 100 degrees C. Which explanation, based on the particle model, best accounts for this evaporation?

    1. A At any moment, water particles have a range of kinetic energies; some surface particles have enough energy to escape the liquid's intermolecular forces as vapour, even far below boiling point.
    2. B The water does not actually evaporate at all; instead, capillary action draws essentially all of it down into the soil and stonework beneath the puddle, and the surface simply appears dry because it has drained away.
    3. C All the water particles must reach exactly 100 degrees C at the surface before any of them gain enough energy to escape as vapour, so evaporation and boiling must always happen at exactly the same temperature.
    4. D Evaporation occurs because heating breaks apart the covalent O-H bonds within each water molecule, releasing separate hydrogen and oxygen gas into the air rather than releasing whole water molecules as vapour.
12. 121 mark

    Molten sodium chloride conducts electricity, but molten (liquid) hexane, a covalent hydrocarbon, does not conduct electricity at all, even though both are liquids at the temperatures concerned. Which explanation correctly accounts for this difference?

    1. A Molten sodium chloride conducts because simply being a liquid always allows charged particles to be spontaneously created within it, whereas hexane is a lighter substance and so cannot generate any charge.
    2. B Both liquids conduct electricity equally well, because any substance that happens to be liquid at the temperature concerned automatically contains enough free charge to carry a current.
    3. C Molten sodium chloride contains free-moving Na+ and Cl- ions that carry charge; molten hexane's neutral covalent molecules have no ions free to move, so it cannot carry a current.
    4. D Molten hexane does not conduct because it has a much lower boiling point than sodium chloride, and any substance with a low boiling point can never contain the charged particles needed to conduct.
13. 131 mark

    Iron can be obtained from iron oxide by heating it with carbon, which removes the oxygen; rock salt, sodium chloride mixed with insoluble clay and sand, can be purified by dissolving it in water, filtering, then evaporating the water. Why does the first process need a chemical reaction, while the second does not?

    1. A Both processes actually involve chemical reactions; dissolving sodium chloride in water is itself supposedly a chemical reaction between the salt and the water, in which entirely new substances are formed as it dissolves.
    2. B Iron oxide requires a chemical reaction because iron is a metal, and separating any metal from any other substance, whatever that substance is and however it happens to be combined with it, always needs a chemical reaction.
    3. C Rock salt needs only a physical method because sodium chloride is nothing more than a mixture of sodium and chlorine atoms sitting near each other, which are not really bonded together in any way at all.
    4. D In iron oxide, iron and oxygen are chemically combined, so a reaction is needed to break the bond and release the metal; in rock salt, the salt is only physically mixed with insoluble impurities, so physical separation is enough.
14. 141 mark

    A student has a solid mixture of iodine, which turns directly from solid to gas on gentle heating without melting, and sand, which remains solid even at high temperatures and does not behave this way. Which technique should be used to separate the iodine from the sand, and why?

    1. A Sublimation: heating turns the iodine directly into a gas without passing through a liquid state, and cooling this gas elsewhere turns it back into solid iodine, leaving the sand unchanged.
    2. B Filtration, because filtration separates any two solids based on differences in particle size, and iodine crystals are small enough to pass through filter paper while the coarser sand grains are retained.
    3. C Crystallisation, because heating the mixture will first dissolve the iodine into the moisture naturally present in the sand, allowing pure iodine to be recovered as crystals once the mixture is left to cool.
    4. D Fractional distillation, because the two solids have different boiling points, so heating the mixture in a fractionating column will separate them into two different liquid fractions collected at different temperatures.
15. 151 mark

    A sample of chlorine gas contains two isotopes, chlorine-35 (about 76 percent) and chlorine-37 (about 24 percent), always in this same fixed ratio wherever it is found naturally. Is chlorine gas correctly classified as an element, a compound, or a mixture?

    1. A A compound, because the two isotopes are present in a fixed ratio, and any substance whose components are combined in a fixed ratio must, by definition, be classified as a chemical compound rather than an element.
    2. B An element: both isotopes have the same number of protons (17) and the same chemical properties, so a fixed ratio of isotopes does not make chlorine gas a compound.
    3. C A mixture, because the two isotopes are physically different types of particle with different masses that happen to be found together in whatever ratio the sample was collected in.
    4. D It cannot be classified using these three terms, because isotopes form their own separate category, distinct from elements, compounds and mixtures, that this classification scheme does not otherwise cover.

Worked solutions

Every question below carries the reasoning, not just the answer. The official material for this test publishes a correct option letter and nothing else.

1. Question 1Answer: B

   1. Ammonium is NH4+, a charge of 1+, and phosphate is PO4 with a charge of 3-.
   2. To balance the overall charge to zero, the criss-cross method gives each ion a subscript equal to the size of the OTHER ion's charge: ammonium gets the subscript 3 (from phosphate's charge of 3), and phosphate gets the subscript 1, which is not written.
   3. This gives (NH4)3PO4, with total charge 3 x 1+ = 3+ from the ammonium balancing 1 x 3- = 3- from the phosphate; the brackets around NH4 are essential, since without them the 3 would appear to apply only to the hydrogen atoms already inside the ion.
   4. The correct formula is therefore (NH4)3PO4, so the answer is B.
   • Why not A: This uses a 1:1 ratio of ammonium to phosphate, which would only balance if the two ions carried equal and opposite charges; ammonium is 1+ and phosphate is 3-, so a 1:1 ratio leaves a charge of 1+ against 3-, which does not balance.
   • Why not C: This applies the criss-cross method the wrong way round, using phosphate's own charge (3) as phosphate's own subscript instead of giving that subscript to ammonium; each ion's subscript should come from the size of the OTHER ion's charge, giving (NH4)3PO4, not NH4(PO4)3.
   • Why not D: This correctly works out that three ammonium ions are needed, but drops the brackets around NH4; without brackets, the subscript 3 appears to multiply only the hydrogen already inside the ion (4 x 3 = 12) rather than the whole NH4 unit, which is exactly why brackets are needed whenever more than one of a polyatomic ion is required.

2. Question 2Answer: A

   1. Melting and boiling are both physical changes that overcome only the weak intermolecular forces between separate water molecules; the O-H covalent bonds within each molecule stay intact throughout both processes.
   2. Melting needs less energy than boiling, because melting only has to loosen the fixed arrangement of a solid, while boiling has to fully separate the molecules from each other against the intermolecular forces holding the liquid together.
   3. Breaking the O-H covalent bonds is a chemical change, not a physical one, and covalent bonds are far stronger than the intermolecular forces overcome in melting or boiling, so this needs by far the most energy of the three processes.
   4. The correct order of increasing energy is therefore melting, then boiling, then breaking the covalent bonds, so the answer is A.
   • Why not B: This treats the covalent bond within each water molecule as needing the least energy to break, when covalent bonds are strong chemical bonds; it is the weak intermolecular forces between separate molecules, overcome during melting and boiling, that need far less energy than breaking a covalent bond.
   • Why not C: This assumes breaking the internal covalent bonds of a molecule takes less energy than fully separating the molecules into a gas by boiling; in reality, breaking a strong covalent bond needs far more energy than overcoming the weak intermolecular forces involved in even boiling.
   • Why not D: This assumes boiling needs less energy than melting, but melting only has to disrupt the ordered arrangement of a solid, whereas boiling must fully separate every molecule against the intermolecular forces holding the liquid together, which needs substantially more energy than melting.

3. Question 3Answer: C

   1. In diamond, every carbon atom forms four strong covalent bonds to four other carbon atoms, extending in three dimensions throughout the whole structure, so there is no plane along which the lattice can easily give way.
   2. In graphite, every carbon atom forms three strong covalent bonds within its own flat layer, but separate layers are held together only by weak intermolecular forces.
   3. Applying a force to graphite can make these weakly-held layers slide over each other with little resistance, which is why graphite is soft and can be used as a lubricant.
   4. Diamond's rigid three-dimensional bonding gives it no such weak plane to slide along, which is why it strongly resists deformation and is extremely hard, so the answer is C.
   • Why not A: Diamond and graphite are both made of exactly the same carbon atoms, with exactly the same atomic mass; the difference in hardness comes from how those atoms are bonded and arranged, not from any difference in atomic mass.
   • Why not B: The covalent bonds within each layer of graphite are just as strong as the covalent bonds in diamond; graphite's softness comes from the weak forces BETWEEN separate layers, not from any weakness in the strong covalent bonds within a layer.
   • Why not D: Both diamond and graphite, in this idealised description, are pure carbon with no other atoms present; the difference in hardness is explained by their different bonding arrangements, a rigid 3-D network against separate weakly-held layers, not by impurities.

4. Question 4Answer: B

   1. Both sodium and magnesium are metals, held together by metallic bonding: a lattice of positive ions surrounded by a sea of delocalised electrons.
   2. Sodium forms Na+ ions and contributes one delocalised electron per atom, while magnesium forms Mg2+ ions and contributes two delocalised electrons per atom.
   3. The greater ionic charge and the greater number of delocalised electrons in magnesium both increase the strength of the electrostatic attraction holding the metallic lattice together.
   4. Because magnesium's metallic bonding is stronger, more energy is needed to overcome it on melting, giving magnesium a much higher melting point than sodium, so the answer is B.
   • Why not A: Atomic mass on its own does not determine melting point; what actually differs between sodium and magnesium is the strength of their metallic bonding, driven by ionic charge and the number of delocalised electrons per atom, not simply how heavy each atom is.
   • Why not C: Both sodium and magnesium are metals held together by metallic bonding, not by covalent or ionic bonds; there is no non-metal present for either ionic or covalent bonding, in the sense described, to apply between the atoms.
   • Why not D: Both sodium and magnesium have metallic structures, not covalent ones; magnesium's higher melting point comes from stronger metallic bonding, due to its ions' greater charge and extra delocalised electrons, not from a change to a covalent lattice.

5. Question 5Answer: D

   1. Each Group 1 metal ion carries the same 1+ charge and contributes one delocalised electron, so charge and electron number do not change down the group.
   2. What does change down the group is the size of the metal ion: each element down the group has an extra electron shell, making the ion larger.
   3. A larger ion places its positive charge further from the surrounding delocalised electrons, weakening the electrostatic attraction between them and so weakening the metallic bonding.
   4. Weaker metallic bonding needs less energy to overcome, giving lower melting points further down the group, so the answer is D.
   • Why not A: Every Group 1 metal has exactly one outer electron; none of them gain extra outer electrons going down the group. Metals are also held together by metallic bonding, not by covalent bonds between individual atoms.
   • Why not B: This reverses the actual trend: Group 1 atoms get LARGER, not smaller, going down the group, as each element has an extra electron shell; it is this increase in ionic size that weakens the metallic bonding and lowers the melting point.
   • Why not C: Reactivity and melting point are different properties, governed by different features of the atoms; there is no general rule that a more reactive metal must have a lower melting point, and this option mistakes a pattern within one group for a universal law.

6. Question 6Answer: C

   1. Reactivity of the halogens (Group 17) decreases going down the group, so the order of reactivity here is chlorine, then bromine, then iodine.
   2. A more reactive halogen can displace a less reactive halide ion from solution, taking its place and releasing the less reactive halogen as the free element.
   3. Bromine is more reactive than iodine, so it displaces iodide ions from potassium iodide solution, forming potassium bromide and free iodine; bromine is less reactive than chlorine, so it cannot displace chloride ions from potassium chloride.
   4. A displacement reaction therefore occurs only with the potassium iodide solution, so the answer is C.
   • Why not A: Reactivity of the halogens decreases going down Group 17. Bromine, below chlorine in the group, cannot displace chloride ions, which belong to the more reactive chlorine; it can only displace halide ions of a less reactive halogen further down the group, such as iodide.
   • Why not B: A halogen displacement reaction is exactly a reaction between one halogen, bromine, as the element, and the halide ion of a different, less reactive halogen, iodide; such reactions are a standard example of displacement and do not require a metal at all.
   • Why not D: This reverses the actual trend: reactivity decreases, not increases, going down Group 17, so bromine, below chlorine and above iodine, can displace the less reactive iodide but not the more reactive chloride; heavier halogens are LESS reactive, not more, so they cannot displace lighter ones.

7. Question 7Answer: A

   1. The sand is insoluble and can be removed straight away by filtration, leaving a filtrate of copper sulfate and ethanol both dissolved in water.
   2. Ethanol and water are miscible, so a separating funnel cannot separate them; because they have different boiling points, fractional distillation is used instead to collect the ethanol, leaving behind the aqueous copper sulfate solution.
   3. The remaining copper sulfate solution can then be crystallised, evaporating off some of the water until crystals of copper sulfate form as the solution cools.
   4. Filtration, then fractional distillation, then crystallisation gives all three pure components in the correct order, so the answer is A.
   • Why not B: The copper sulfate is dissolved, not present as solid crystals, so it cannot simply be filtered out while it remains in solution; crystals only form once enough water has been removed from the solution, which happens after the ethanol has been distilled off, not before.
   • Why not C: A separating funnel is used for immiscible liquids that form two distinct layers, such as oil and water; ethanol and water are miscible and mix completely into one layer, so a separating funnel cannot separate them, and fractional distillation is needed instead.
   • Why not D: Crystallisation cannot be carried out on the whole mixture at once: the insoluble sand and the volatile ethanol would both still be present and would interfere with the process; the sand must be filtered out and the ethanol distilled off first, before crystallisation is used on the remaining copper sulfate solution.

8. Question 8Answer: D

   1. Rf values are only comparable when measured under identical conditions, which is why the reference dye was run alongside the ink using the same solvent, paper and temperature.
   2. The ink separating into three spots shows it is a mixture of at least three different substances, since a pure substance produces only a single spot.
   3. One of those three spots has the same Rf value as the pure reference dye under these matching conditions, which is evidence that this component of the ink is the same substance as the reference dye.
   4. The ink is therefore a mixture containing the reference dye alongside at least two other substances, so the answer is D.
   • Why not A: The number of spots simply reflects however many different substances happen to be present, not a fixed rule of three; producing three spots shows the ink is a mixture of several components, which is the opposite of being pure.
   • Why not B: It is the ink, not the reference dye, that separates into three spots; the reference dye itself gives just one spot, and one of the ink's three spots matching the reference dye's Rf value under identical conditions is exactly the evidence that the ink CONTAINS that dye, not evidence against it.
   • Why not C: Comparing Rf values measured under identical conditions, same solvent, paper and temperature, is precisely how chromatography is used to identify substances; a matching Rf value under the same conditions is good evidence, though not absolute proof, that two spots are the same substance.

9. Question 9Answer: B

   1. Ionic bonding occurs between a metal and a non-metal, through complete transfer of electrons; covalent bonding occurs between two non-metals, through sharing of electrons.
   2. Calcium, potassium and magnesium are all metals, so each of their pairings with a non-metal, chlorine or oxygen, forms ionic bonds, not covalent bonds.
   3. Carbon and chlorine are both non-metals, so when they react they share electrons between their atoms rather than transferring them completely, forming covalent bonds.
   4. Carbon and chlorine is therefore the pair that forms covalent bonds, so the answer is B.
   • Why not A: This description of calcium and chlorine forming Ca2+ and Cl- ions is accurate, but it describes IONIC bonding through electron transfer; the question asks for a pair that forms COVALENT bonds, which this pairing does not.
   • Why not C: The unequal charges described here are exactly what fixes the ratio of ions in the ionic compound formed, potassium oxide, K2O; potassium (a metal) and oxygen (a non-metal) still bond ionically by complete electron transfer, just not in a 1:1 ratio, so this is not a reason for covalent bonding.
   • Why not D: Magnesium is in Period 3, but oxygen is in Period 2, so this claim is factually wrong; more importantly, whether bonding is ionic or covalent depends on whether the elements are metals or non-metals, not on which period they are in, and magnesium (a metal) with oxygen (a non-metal) actually bond ionically, forming Mg2+ and O2- ions.

10. Question 10Answer: C

    1. Each halogen atom has seven outer-shell electrons and needs to gain just one more to complete a full outer shell, forming a stable negative ion.
    2. Going down Group 17, atoms become larger as extra electron shells are added, and the increasing number of inner shells shields the outer shell more from the attraction of the nucleus.
    3. This means the nucleus attracts an incoming electron less strongly further down the group, so it becomes harder for the atom to gain an electron.
    4. Reactivity therefore decreases going down the group because of increasing atomic radius and shielding, so the answer is C.
    • Why not A: Molecular mass on its own does not determine reactivity; what matters is how strongly the nucleus can attract an incoming electron, which depends on atomic radius and shielding, not simply how heavy the molecule is.
    • Why not B: Every halogen atom has the same number of outer-shell electrons (seven), regardless of its position in the group, and each only ever needs to gain ONE electron to complete its outer shell; this does not change going down the group.
    • Why not D: The strength of the covalent bond within each diatomic molecule does not directly control how reactive that molecule is towards other substances; reactivity here depends on how easily each halogen ATOM can attract and gain an extra electron, which is governed by atomic radius and shielding, not by the internal bond.

11. Question 11Answer: A

    1. Particles in a liquid do not all have the same kinetic energy; at any moment, some particles have more energy than the average and some have less.
    2. At the surface of the liquid, a particle with enough kinetic energy can overcome the intermolecular forces holding it in the liquid and escape as vapour, even if the liquid as a whole is far below its boiling point.
    3. This is different from boiling, which needs the whole liquid to reach the temperature at which bubbles of vapour can form throughout it, not just at the surface.
    4. Evaporation from the surface therefore continues at any temperature below boiling point, which is why the puddle disappears on a warm day, so the answer is A.
    • Why not B: This proposes an unrelated mechanism and ignores the genuine physical explanation for evaporation; while some water may seep into certain surfaces, a puddle disappearing in the open on a warm day is explained by evaporation of liquid water into water vapour.
    • Why not C: This confuses evaporation, which can occur at any temperature below the boiling point from surface particles with above-average energy, with boiling, which requires the WHOLE liquid to reach the temperature at which its vapour pressure equals atmospheric pressure; evaporation does not need every particle, or the bulk liquid, to reach the boiling point.
    • Why not D: Evaporation is a physical change of state, not a chemical change; the covalent O-H bonds within each water molecule stay intact, and only the weaker intermolecular forces between separate water molecules are overcome when a molecule escapes into the vapour phase.

12. Question 12Answer: C

    1. Sodium chloride is an ionic compound: even when molten, it consists of freely moving Na+ and Cl- ions, which can carry an electric current through the liquid.
    2. Hexane is a covalent compound made of individual neutral molecules; melting or boiling it does not create any ions or free electrons, so there is nothing charged to carry a current.
    3. The presence of freely moving charged particles is what allows a substance to conduct electricity, regardless of whether it is solid, liquid or gas.
    4. Because molten sodium chloride has free ions and molten hexane does not, only the sodium chloride conducts electricity, so the answer is C.
    • Why not A: Being a liquid does not by itself create charged particles; whether a liquid conducts depends on whether it already contains freely moving ions, as with molten ionic sodium chloride, or is made of neutral molecules with no ions at all, as with molten hexane.
    • Why not B: Conductivity is not a general property of all liquids; it depends entirely on whether free-moving charged particles, ions, or delocalised electrons as in a metal, are present, which molten hexane's neutral covalent molecules do not provide.
    • Why not D: Boiling point is unrelated to whether a substance conducts electricity; conductivity depends on the presence of mobile charged particles, not on how high or low a substance's boiling point happens to be.

13. Question 13Answer: D

    1. In iron oxide, iron and oxygen atoms are chemically bonded together as a compound, so simply dissolving, filtering or evaporating cannot separate them; a chemical reaction with carbon is needed to break this bond and release the iron.
    2. In rock salt, the sodium chloride itself is a compound too, but it is only physically mixed with insoluble sand and clay, not chemically bonded to them.
    3. Because the salt and the impurities are only physically mixed, physical processes, dissolving the soluble salt, filtering out the insoluble solids, then evaporating to recover the salt, are enough to separate them.
    4. A chemical reaction is needed wherever a chemical bond has to be broken to obtain an element, but only physical separation is needed where substances are simply mixed together, so the answer is D.
    • Why not A: Dissolving a soluble solid in water is a physical process; the sodium chloride separates into hydrated ions but no new substance is formed, and the process is fully reversible by evaporation, unlike a chemical reaction, which forms new substances.
    • Why not B: Metals chemically combined with non-metals, as in iron oxide, do need a chemical reaction to be extracted; but a metal compound only physically mixed with other substances, as in rock salt contaminated with sand and clay, can be separated by physical means alone. It is how the substances are combined, not simply whether a metal is present, that decides which method is needed.
    • Why not C: Sodium chloride IS a compound: sodium and chlorine are chemically bonded together ionically within each salt crystal. It is the impurities, such as sand and clay, that are only physically mixed with the salt, which is why the impurities, not the salt itself, can be removed by physical separation.

14. Question 14Answer: A

    1. Iodine is unusual in that gently heating it turns it directly into a purple gas without first melting into a liquid; this process is called sublimation.
    2. Sand does not behave this way and stays solid at the temperatures used, so heating the mixture drives off only the iodine, as a gas, leaving the sand behind.
    3. The gaseous iodine can then be cooled elsewhere in the apparatus, where it turns directly back into a solid, now separated from the sand.
    4. Sublimation is therefore the correct technique for separating a substance that sublimes from a solid that does not, so the answer is A.
    • Why not B: Filtration works by trapping a solid suspended in a liquid that passes through the filter paper; it does not act as a dry sieve for two solids mixed with no liquid present, so neither iodine nor sand would pass through regardless of particle size, and there is nothing here for filtration to separate.
    • Why not C: Crystallisation recovers a dissolved solid from a solution by evaporating the solvent, but there is no solvent here and nothing has been dissolved; iodine does not need to be dissolved at all, since it can be separated directly by heating.
    • Why not D: Fractional distillation separates miscible LIQUIDS with different boiling points; sand and iodine are both solids, and iodine does not melt into a liquid before turning to gas, so there are no boiling points to compare here.

15. Question 15Answer: B

    1. Isotopes are atoms of the same element that have the same number of protons but different numbers of neutrons, so chlorine-35 and chlorine-37 are both still chlorine, with 17 protons each.
    2. An element is defined as a substance made of only one type of atom, meaning only one proton number; having atoms with different neutron numbers, isotopes, does not change this.
    3. A compound would require chlorine to be chemically bonded to a different element, and a mixture would require genuinely different substances mixed together; neither applies here, since both isotopes are chlorine.
    4. Chlorine gas containing a fixed ratio of two isotopes is therefore still correctly classified as an element, so the answer is B.
    • Why not A: A fixed ratio between components is necessary but not sufficient to define a compound; a compound must also be formed of two or more DIFFERENT ELEMENTS chemically bonded together. Chlorine-35 and chlorine-37 are both the same element, chlorine, 17 protons each, just with a different number of neutrons, so this is not a compound.
    • Why not C: The two isotopes are not different substances or different elements; they are both chlorine atoms with the same proton number and the same chemical behaviour, just a different number of neutrons. A mixture requires two or more different substances, which isotopes of the same element are not.
    • Why not D: Every element exists as one or more isotopes; being composed of isotopes does not place a substance into some separate fourth category. Chlorine gas is still correctly classified as an element, since it consists of only one type of atom, defined by its proton number.