ESAT Chemistry: Bonding, structure and the groups, set 2

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

1. 11 mark

   Brass is a metal alloy made by mixing molten copper and zinc together and allowing the mixture to cool. There is no fixed ratio of the two metals used, and each metal keeps its own individual properties within the solid. Which term correctly classifies brass?

   1. A A compound, because heating always joins two substances into one new chemical species.
   2. B A mixture, since copper and zinc are physically combined without a fixed chemical ratio.
   3. C An element, since alloys made only from metals count as a single pure metallic substance.
   4. D An isotope, since combining two metallic elements always produces isotopes of each other.
2. 21 mark

   A magnesium atom (electron configuration 2,8,2) reacts with an oxygen atom (electron configuration 2,6) to form the ionic compound magnesium oxide. How many electrons does each magnesium atom transfer to each oxygen atom, and what is the resulting formula?

   1. A 1 electron is transferred from each magnesium atom, forming Mg+ ions and the formula Mg2O.
   2. B 2 electrons are transferred, forming Mg2+ and O2-, giving the formula MgO2.
   3. C 8 electrons are transferred, filling oxygen's outer shell completely, giving the formula MgO8.
   4. D 2 electrons are transferred, forming Mg2+ and O2-, giving the formula MgO.
3. 31 mark

   Iron(III) sulfate is formed from Fe3+ ions and sulfate ions (SO4, each carrying a charge of 2-). The Roman numeral (III) shows that the iron ion here carries a charge of 3+. What is the correct formula for iron(III) sulfate?

   1. A Fe2(SO4)3, two iron ions for every three sulfate ions
   2. B FeSO4, one iron ion for every one sulfate ion
   3. C Fe3(SO4)2, three iron ions for every two sulfate ions
   4. D Fe2SO4, two iron ions for every one sulfate ion
4. 41 mark

   Calcium ions, Ca2+ (charge 2+), react with hydroxide ions, OH (charge 1-), to form calcium hydroxide. What is the correct formula for calcium hydroxide?

   1. A CaOH, one calcium atom, one oxygen atom and one hydrogen atom
   2. B CaOH2, one calcium atom, one oxygen atom and two hydrogen atoms
   3. C Ca(OH)2, one calcium ion and two complete hydroxide ions
   4. D Ca2OH, two calcium atoms, one oxygen atom and one hydrogen atom
5. 51 mark

   Graphite is a giant covalent structure made entirely of carbon atoms, arranged in layers. Unlike diamond (also a giant covalent structure of carbon), graphite conducts electricity. What is the best explanation for this difference?

   1. A Graphite contains metal impurities trapped between its layers, and it is these delocalised metal electrons that carry the charge.
   2. B Each carbon atom in graphite forms only 3 covalent bonds, leaving one outer electron per atom delocalised and free to move.
   3. C The layers in graphite are held together by ionic bonds, and it is the mobile ions between the layers that carry the charge.
   4. D Graphite is not really a giant covalent structure at all; it is a mixture of separate carbon molecules, which conduct electricity like a liquid.
6. 61 mark

   Metals are malleable, meaning they can be hammered or rolled into shape without shattering. What is the best explanation for this property, in terms of metallic structure and bonding?

   1. A The covalent bonds between neighbouring metal atoms are weak and easily broken, allowing the atoms to separate and reform in a new shape.
   2. B Metal ions are held together only by weak intermolecular forces, which break easily under stress and allow the metal to bend.
   3. C Metals contain small gaps between fixed ions, and hammering simply squeezes these gaps smaller without any ions moving.
   4. D The layers of positive metal ions slide over one another under stress, while delocalised electrons keep the metallic bonding intact.
7. 71 mark

   Solid sodium chloride does not conduct electricity, but molten sodium chloride (and sodium chloride dissolved in water) does conduct electricity. What is the best explanation for this difference?

   1. A Solid sodium chloride is held together by covalent bonds, while molten sodium chloride is held together by ionic bonds.
   2. B Molten sodium chloride contains delocalised electrons, in the same way that a metal does, and it is these electrons that carry the current.
   3. C In the solid, the ions are held in fixed positions and cannot move to carry charge; melting or dissolving frees them to move.
   4. D Solid sodium chloride contains no charged particles at all; ions are only created once the sodium chloride melts or dissolves.
8. 81 mark

   When water vapour in the air condenses on a cold window pane to form liquid water droplets, what happens to the arrangement of the particles and to the energy of the system?

   1. A The particles move closer together and slow down as stronger intermolecular forces form, releasing energy to the surroundings.
   2. B The particles move further apart and speed up as the intermolecular forces between them weaken, and energy is absorbed from the surroundings.
   3. C The particles move closer together and slow down, but no energy is transferred at all, since condensation is simply a change of appearance.
   4. D The arrangement and speed of the particles stay exactly the same; only the visible state changes from gas to liquid.
9. 91 mark

   Going down Group 17 from chlorine to iodine, the halogens change physical state at room temperature: chlorine is a gas, bromine is a liquid and iodine is a solid. What is the best explanation for this trend?

   1. A The covalent bond within each diatomic molecule (e.g. within each Br2 or I2 molecule) becomes stronger going down the group, holding the molecules more rigidly in place.
   2. B The molecules become smaller and lighter going down the group, so they escape more easily as a solid rather than a gas.
   3. C The halogens become more chemically reactive going down the group, and more reactive substances naturally have higher melting and boiling points.
   4. D The weak intermolecular forces between separate molecules become stronger as the molecules get larger (with more electrons), so more energy is needed to separate them.
10. 101 mark

    The noble gases (Group 18) all exist as single, separate atoms (not molecules). Going down the group from helium to xenon, their boiling points increase. What is the best explanation for this trend?

    1. A Reactivity increases going down the group, and increasing reactivity always raises an element's boiling point.
    2. B As the atoms get larger going down the group, the weak intermolecular forces between them become stronger, needing more energy to separate them.
    3. C The covalent bonds between neighbouring noble gas atoms become stronger going down the group, holding the atoms together more tightly.
    4. D The atoms become lighter going down the group, needing more energy to be held together as a liquid instead of escaping as a gas.
11. 111 mark

    Small, freshly cut pieces of lithium, sodium and potassium are each added separately to water. Which statement correctly ranks how vigorously they react, from least to most vigorous, and gives the correct reason?

    1. A Lithium reacts least vigorously, then sodium, then potassium most vigorously, as reactivity increases going down Group 1.
    2. B Potassium reacts least vigorously, then sodium, then lithium most vigorously, because reactivity decreases going down Group 1.
    3. C All three metals react equally vigorously, because they all belong to Group 1 and so all have one electron in their outer shell.
    4. D Lithium reacts least vigorously, then sodium, then potassium most vigorously, because potassium atoms are smaller and so more of them react at once.
12. 121 mark

    Iodine solution is added to an aqueous solution of potassium chloride. What would be observed, and why?

    1. A The solution turns orange/brown, because iodine is more reactive than chlorine lower down the group, where atoms are larger and more reactive.
    2. B A white precipitate forms, because potassium iodide is insoluble and falls out of solution.
    3. C No visible reaction occurs, because iodine is less reactive than chlorine and so cannot displace chloride ions from solution.
    4. D The solution turns colourless, because iodine reacts with the chloride ions to form a new, colourless compound.
13. 131 mark

    A student has a mixture of vegetable oil and water, which do not mix and settle into two separate layers, with the less dense oil floating on top. Which technique should be used to separate the two liquids?

    1. A Fractional distillation, because it separates any two liquids according to their different boiling points.
    2. B Filtration, because a filter paper separates any mixture into its separate solid and liquid components.
    3. C Evaporation, because heating the mixture will boil off one of the two liquids, leaving the other behind.
    4. D A separating funnel, which runs off the denser liquid from the bottom, leaving the less dense liquid behind.
14. 141 mark

    A liquid contains a very fine, insoluble solid, so finely divided that it passes straight through filter paper rather than being caught by it. Which separation technique should be used instead of filtration?

    1. A Chromatography, which separates substances in a mixture according to how far they travel through a stationary material.
    2. B Centrifugation, which spins the mixture at high speed so that the denser solid particles separate out and settle.
    3. C Crystallisation, which recovers a dissolved solid by evaporating off the solvent until crystals form.
    4. D Simple distillation, which separates a solvent from a dissolved solute by boiling off and condensing the solvent.
15. 151 mark

    A student measures an Rf value of 0.45 for a coloured spot in their own chromatography experiment, but a data book lists an Rf value of 0.62 for the same substance under different published conditions. The student concludes that their sample must be a different, unknown substance. Is this conclusion justified?

    1. A No, because Rf values depend on the exact experimental conditions used, so a fair comparison needs the reference run alongside the unknown.
    2. B Yes, because Rf values are fixed constants for a given substance and can be compared directly, whatever the conditions used to measure them.
    3. C Yes, because any difference at all between two measured Rf values, however small, always proves the substances are chemically different.
    4. D No, because Rf values cannot be used to compare or identify substances at all; they can only be used to check whether a single sample is pure.

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. A mixture contains two or more substances that are not chemically bonded together, and whose relative proportions can vary.
   2. Brass contains copper atoms and zinc atoms mixed together, with no fixed ratio and no chemical bond joining copper atoms to zinc atoms.
   3. Because each metal keeps its own separate identity and the proportions are not fixed, brass fits the definition of a mixture rather than a compound, an element or an isotope.
   4. Brass is therefore a mixture, so the answer is B.
   • Why not A: Heating two substances together does not automatically create a new chemical compound; a compound requires atoms of different elements to bond chemically in a fixed ratio, which brass does not have between its copper and zinc.
   • Why not C: An alloy made from two different metals is not a pure element just because it has a shiny, metallic look; brass actually contains both copper atoms and zinc atoms mixed together, not one single type of atom.
   • Why not D: Isotopes are different atoms of the SAME element that differ only in neutron number; combining two different metallic elements does not turn them into isotopes of one another, since copper and zinc remain two separate elements.

2. Question 2Answer: D

   1. Magnesium's outer shell configuration ends in 2, so a magnesium atom loses its 2 outer electrons to reach the electron configuration of neon (2,8), forming a 2+ ion.
   2. Oxygen's outer shell configuration ends in 6, so an oxygen atom gains 2 electrons to reach a full outer shell of 8 (also the configuration of neon), forming a 2- ion.
   3. Since the magnesium ion has a charge of 2+ and the oxide ion has a charge of 2-, these two charges balance exactly in a 1:1 ratio, with no extra ions of either kind needed.
   4. The formula of magnesium oxide is therefore MgO, so the answer is D.
   • Why not A: Magnesium is in Group 2, so it must lose 2 electrons (not just 1) from its outer shell to reach the electron configuration of neon; treating it as if it loses only 1 electron, like a Group 1 metal, gives the wrong charge and the wrong formula.
   • Why not B: This correctly finds the charges on the two ions (Mg2+ and O2-) but then fails to balance them: since the charges are equal in size (2+ and 2-), one magnesium ion balances exactly one oxide ion, giving a 1:1 ratio, MgO, not MgO2.
   • Why not C: This confuses the total number of electrons that fit in a full outer shell (8, for oxygen's second shell overall) with the number of electrons a single magnesium atom actually needs to lose (2) to empty its own outer shell down to a stable, full shell underneath.

3. Question 3Answer: A

   1. The Roman numeral (III) tells us the iron ion here is Fe3+, carrying a charge of 3+, and the sulfate ion, SO4, carries a charge of 2-.
   2. Using the criss-cross method, the subscript for iron comes from the size of the sulfate ion's charge (2), and the subscript for sulfate comes from the size of the iron ion's charge (3).
   3. This gives 2 iron ions for every 3 sulfate ions: a total positive charge of 2 x 3+ = 6+ balances a total negative charge of 3 x 2- = 6-, and the sulfate group must be shown in brackets since more than one is needed.
   4. The correct formula is therefore Fe2(SO4)3, so the answer is A.
   • Why not B: This treats iron as if it only ever forms a 2+ ion, as in iron(II) sulfate; the Roman numeral (III) in the name specifically tells us the iron ion here is Fe3+, not Fe2+, and using the wrong charge gives the wrong ratio of ions.
   • Why not C: This uses each ion's OWN charge as its OWN subscript instead of crossing the charges over; the subscript on iron should come from the SIZE of the sulfate ion's charge (2), and the subscript on sulfate should come from the size of the iron ion's charge (3), which is the opposite way round from this option.
   • Why not D: This correctly works out that iron and sulfate combine in a 2:3 ratio, but forgets to show the sulfate ion is tripled; without brackets and a subscript of 3 outside them, Fe2SO4 represents only one sulfate ion for every two iron ions, which does not balance the charges.

4. Question 4Answer: C

   1. Calcium ions carry a charge of 2+ and hydroxide ions carry a charge of 1-, so more hydroxide ions than calcium ions are needed to balance the overall charge to zero.
   2. Using the criss-cross method, the subscript for calcium comes from the size of hydroxide's charge (1), and the subscript for hydroxide comes from the size of calcium's charge (2).
   3. This gives one calcium ion for every two hydroxide ions; because hydroxide is itself a group of two atoms (O and H), brackets are needed around OH before writing the subscript 2, to show the whole ion is doubled.
   4. The correct formula is therefore Ca(OH)2, so the answer is C.
   • Why not A: This assumes a simple 1:1 ratio of ions, but calcium's charge (2+) is twice the size of hydroxide's charge (1-); one calcium ion needs two hydroxide ions to balance its charge, not just one.
   • Why not B: This correctly finds that two hydroxide ions are needed, but without brackets around OH, the subscript 2 appears to apply only to the oxygen atom already written, not to the whole OH group; brackets are needed to show that TWO complete hydroxide ions are present.
   • Why not D: This uses calcium's own charge (2) as calcium's own subscript instead of crossing the charges over; the subscript on calcium should come from the size of hydroxide's charge (1), giving just one calcium ion, while the subscript on hydroxide should come from the size of calcium's charge (2).

5. Question 5Answer: B

   1. In diamond, each carbon atom forms 4 covalent bonds to 4 other carbon atoms, using all 4 of its outer electrons in bonding, so there are no free electrons and diamond does not conduct electricity.
   2. In graphite, each carbon atom forms only 3 covalent bonds, to 3 other carbon atoms within its own layer, using only 3 of its 4 outer electrons.
   3. This leaves one outer electron per carbon atom delocalised, free to move within the layer, although the layers themselves are held together only by weak intermolecular forces.
   4. These delocalised electrons are what allow graphite to conduct electricity along its layers, so the answer is B.
   • Why not A: Graphite is a pure element made only of carbon atoms; it contains no metal impurities. Its ability to conduct comes from carbon's own bonding arrangement (each atom using only 3 of its 4 outer electrons in bonds), not from any other element being present.
   • Why not C: The forces between the layers in graphite are weak intermolecular (van der Waals) forces, not ionic bonds, which is why the layers can slide over each other easily; there are no mobile ions in graphite at all, since it contains no charged particles.
   • Why not D: Graphite is a genuine giant covalent structure: every carbon atom is joined by strong covalent bonds to three neighbouring carbon atoms within each layer, forming one continuous network, not separate small molecules.

6. Question 6Answer: D

   1. In a metal, positive ions are arranged in layers, surrounded by a sea of delocalised electrons that are not fixed to any one ion.
   2. When a force is applied, one layer of ions can slide over the layer next to it into a new position.
   3. Because the delocalised electrons are free to move and are not tied to particular ions, they simply redistribute around the new arrangement, so the metallic bond is not broken as the layers slide.
   4. This ability of layers to slide without breaking the overall metallic bonding is what makes metals malleable, so the answer is D.
   • Why not A: Metallic bonding is not covalent bonding; there are no discrete bonds between individual pairs of metal atoms to break and reform. Whole layers of ions slide as a block, while the delocalised electron sea adjusts to hold the new arrangement together.
   • Why not B: Metallic bonding is actually a strong form of bonding (the electrostatic attraction between positive ions and delocalised electrons), not a weak intermolecular force; if metallic bonds were weak in this way, metals would generally have low melting points, but most have high melting points.
   • Why not C: This describes malleability as if no ions move at all, only gaps shrinking; in reality, whole layers of ions slide past each other into new positions when a metal is bent or hammered, which is why the shape permanently changes.

7. Question 7Answer: C

   1. Sodium chloride is an ionic compound, made up of Na+ and Cl- ions held together by strong electrostatic forces in a giant lattice, in both its solid and molten forms.
   2. In the solid state, these ions are held tightly in fixed positions in the lattice and cannot move from place to place, so no charge can flow and the solid does not conduct.
   3. When sodium chloride melts (or dissolves in water), the rigid lattice breaks down and the ions become free to move throughout the liquid.
   4. These now-mobile ions can carry charge from place to place, so molten (or dissolved) sodium chloride conducts electricity, and the answer is C.
   • Why not A: Sodium chloride is held together by ionic bonds in both the solid and molten states; melting does not change the type of bonding present, only whether the ions are free to move.
   • Why not B: Conducting electricity via delocalised electrons is the mechanism used in metallic bonding; molten ionic compounds conduct by a different mechanism, the movement of whole charged ions through the liquid, not free electrons.
   • Why not D: Solid sodium chloride already consists entirely of Na+ and Cl- ions, held in a fixed lattice arrangement; melting or dissolving does not create these ions, it only frees ions that were already there to move.

8. Question 8Answer: A

   1. In water vapour (a gas), particles are far apart and move quickly in random directions, with very weak forces acting between them.
   2. As the vapour condenses into liquid water, the particles move much closer together, allowing stronger intermolecular forces of attraction to act between them.
   3. Forming these stronger attractions between particles releases energy to the surroundings, which is why condensation is an exothermic (energy-releasing) change, unlike melting, boiling and evaporating, which absorb energy.
   4. The particles therefore move closer together and slow down, releasing energy, so the answer is A.
   • Why not B: This describes what happens when a liquid evaporates into a gas (particles moving apart and speeding up, absorbing energy), not the reverse process of a gas condensing into a liquid, which is what is described here.
   • Why not C: Condensation is not simply a change of appearance with no energy transfer; forming stronger intermolecular attractions as particles move closer together releases energy to the surroundings, which is why condensation is described as an exothermic change.
   • Why not D: Condensation involves a real physical change in how closely particles are packed and how much kinetic energy they have, not just a change of label; gas particles are far apart and fast moving, while liquid particles are much closer together and slower.

9. Question 9Answer: D

   1. Chlorine, bromine and iodine all exist as simple diatomic molecules (Cl2, Br2, I2), held together internally by a strong covalent bond, with only weak intermolecular forces acting between separate molecules.
   2. Going down the group, each successive halogen atom has more electrons and more electron shells, making the molecules larger.
   3. Larger molecules with more electrons experience stronger intermolecular (van der Waals) forces between them, so more energy is needed to separate the molecules and allow them to move freely.
   4. This is why chlorine (weakest intermolecular forces) is a gas, bromine is a liquid, and iodine (strongest intermolecular forces) is a solid at room temperature, so the answer is D.
   • Why not A: This confuses the covalent bond WITHIN each diatomic molecule (which is not what changes on melting or boiling) with the much weaker intermolecular forces BETWEEN separate molecules (which must be overcome); it is the intermolecular forces, not the internal covalent bond, that determine the physical state.
   • Why not B: This gets the size trend backwards: going down Group 17, the halogen molecules get larger and heavier (more protons, neutrons and electrons per atom), not smaller and lighter, which is part of why they become harder, not easier, to vaporise.
   • Why not C: Reactivity of the halogens actually decreases going down the group, not increases (chlorine is more reactive than bromine, which is more reactive than iodine); reactivity and physical state are also separate properties governed by different factors.

10. Question 10Answer: B

    1. The noble gases exist as single, separate atoms, with only weak intermolecular (van der Waals) forces acting between neighbouring atoms, since there is no covalent bonding between them.
    2. Going down the group from helium to xenon, each atom has more electrons and more occupied shells, making the atoms larger.
    3. Larger atoms with more electrons experience stronger intermolecular forces between them, so more energy is needed to separate the atoms and allow them to move freely as a gas.
    4. This is why the boiling points of the noble gases increase going down the group, so the answer is B.
    • Why not A: The noble gases are all extremely unreactive at every point in the group (having full outer shells is their defining property), so there is no meaningful reactivity trend to link to boiling point here; the two properties are governed by separate factors.
    • Why not C: Noble gas atoms exist as separate, individual atoms; there are no covalent bonds between one noble gas atom and its neighbour at all, only weak intermolecular (van der Waals) forces, so there is no covalent bond that could get stronger.
    • Why not D: This gets the mass trend backwards: going down Group 18, each noble gas atom has more protons, neutrons and electrons and so becomes heavier, not lighter, which is part of why the intermolecular forces (and so the boiling point) increase.

11. Question 11Answer: A

    1. Each Group 1 metal has one electron in its outer shell, and reacting with water means losing that single electron to form a 1+ ion.
    2. Going down the group from lithium to sodium to potassium, each element gains an extra electron shell, so the outer electron sits further from the positive nucleus and is shielded by more inner shells.
    3. Because this outer electron is progressively easier to lose going down the group, the reaction with water becomes progressively more vigorous.
    4. Lithium therefore reacts least vigorously, sodium more vigorously, and potassium most vigorously of the three, so the answer is A.
    • Why not B: This reverses the correct trend: reactivity of the alkali metals actually increases going down Group 1 (as extra electron shells make the outer electron easier to lose), so potassium reacts most vigorously and lithium least, not the other way round.
    • Why not C: Having the same number of outer electrons (one) explains why all three metals react in a similar WAY (losing one electron to form a 1+ ion), but it does not mean the reactions happen equally vigorously; the ease of losing that electron still changes down the group.
    • Why not D: This gives the correct order of vigour (lithium least, potassium most) but the wrong reason: potassium atoms are actually LARGER than lithium atoms (having more electron shells), not smaller, and the correct reason is that potassium's outer electron is further from the nucleus and more easily lost, not a difference in atom size.

12. Question 12Answer: C

    1. Reactivity of the halogens (Group 17) decreases going down the group, so chlorine is more reactive than bromine, which is more reactive than iodine.
    2. In a displacement reaction, a MORE reactive halogen can displace a LESS reactive halide ion from solution; here, iodine is LESS reactive than chlorine, so it cannot displace chloride ions from potassium chloride.
    3. With no reaction able to take place, none of the coloured, free halogen is released into solution, and no new compound or precipitate forms.
    4. The result is no visible reaction, so the answer is C.
    • Why not A: This reverses the Group 17 trend: reactivity of the halogens decreases going down the group (chlorine is more reactive than bromine, which is more reactive than iodine), because the incoming electron needed to form a halide ion is added further from the nucleus and less strongly attracted, lower down the group.
    • Why not B: This confuses displacement with a precipitation reaction, such as the silver nitrate test for halide ions; potassium iodide is actually soluble in water, so even if it were formed, it would not appear as a precipitate.
    • Why not D: Iodine does not react with chloride ions at all here, since iodine is less reactive than chlorine and cannot displace it; with no reaction taking place, the colour of the iodine solution would remain, not disappear.

13. Question 13Answer: D

    1. Oil and water are immiscible liquids: they do not mix together and instead settle into two separate layers, with the less dense liquid (oil) floating on top of the denser liquid (water).
    2. A separating funnel has a tap at the bottom, which allows the lower, denser layer to be run off and collected separately, while the upper, less dense layer stays behind in the funnel.
    3. This physically separates the two liquids from each other without needing to boil, filter or evaporate anything.
    4. A separating funnel is therefore the correct technique for separating oil and water, so the answer is D.
    • Why not A: Fractional distillation is used to separate MISCIBLE liquids that mix completely into a single layer, based on their different boiling points; oil and water are immiscible and already form two separate layers, so there is no need to heat and boil them apart.
    • Why not B: Filtration separates an insoluble SOLID from a liquid using a filter paper; both oil and water here are liquids, and neither would be caught by a filter paper, so filtration cannot separate them.
    • Why not C: Evaporation is used to recover a dissolved SOLID from a solution by boiling off the solvent; here there are two immiscible liquids, both of which are wanted, not a solid dissolved in one liquid, so evaporation is not an appropriate method.

14. Question 14Answer: B

    1. The solid here is insoluble but so finely divided that its particles are small enough to pass straight through the pores of filter paper, so simple filtration will not remove it.
    2. Centrifugation spins the mixture at very high speed, and this force pushes the denser solid particles outward and downward, causing them to separate out and settle at the bottom of the tube.
    3. This works even for particles too fine for filter paper, because it relies on differences in density under a strong spinning force, rather than the physical size of the pores in a filter.
    4. Centrifugation is therefore the correct technique here, so the answer is B.
    • Why not A: Chromatography separates substances that are all dissolved (or dispersed) in a moving solvent, based on how strongly each one interacts with the stationary material as it travels; it is not a technique for removing a fine insoluble solid from a liquid.
    • Why not C: Crystallisation recovers a solid that is DISSOLVED in a solution, by evaporating the solvent until crystals form; the solid described here is insoluble (not dissolved), so there is no solution to evaporate in the first place.
    • Why not D: Simple distillation separates a dissolved solute from its solvent by boiling off the solvent as vapour and condensing it elsewhere; it is designed for a dissolved solute, not an insoluble solid suspended in the liquid.

15. Question 15Answer: A

    1. An Rf value is not an absolute constant for a substance; it depends on the exact experimental conditions used, including the solvent, the type of paper and the temperature.
    2. Two chromatography experiments run under different conditions can give different Rf values for the very same substance, simply because the conditions themselves differ.
    3. To reliably identify or compare a substance using Rf values, the unknown and a reference sample must be run side by side, on the same paper, in the same solvent, at the same time.
    4. Because the student's value and the data book value come from different conditions, the difference alone does not justify concluding the sample is a different substance, so the answer is A.
    • Why not B: Rf values are not fixed constants independent of conditions; the same substance can give a different Rf value in a different solvent, on different paper, or at a different temperature, so a value from a data book cannot be safely compared to a home experiment run under different conditions.
    • Why not C: Small differences in Rf value can easily arise just from differences in experimental conditions between two separate experiments, rather than proving the substances are chemically different; a difference this size, measured under different conditions, is not strong enough evidence on its own.
    • Why not D: Rf values can be used both to help identify unknown substances (usually by comparing spots run side by side under identical conditions) and to assess a sample's purity (by counting the number of spots produced); this option incorrectly restricts chromatography to only one of its two common uses.