ESAT Chemistry: Chemical analysis and the atmosphere, set 2

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

1. 11 mark

   Dilute sulfuric acid is electrolysed using inert electrodes. The gas collected at the anode (positive electrode) is tested by inserting a glowing splint into the gas jar, and the splint relights.

   Which gas has been collected at the anode?

   1. A Hydrogen
   2. B Carbon dioxide
   3. C Oxygen
   4. D Chlorine
2. 21 mark

   An unknown gas is bubbled through water to form a solution. A strip of damp blue litmus paper is dipped into this solution: the paper turns red immediately, and within a few seconds it loses all colour, becoming white.

   Which gas was bubbled through the water?

   1. A Chlorine
   2. B Sulfur dioxide
   3. C Hydrogen chloride
   4. D Carbon dioxide
3. 31 mark

   A student is testing an unknown solution for the presence of a halide ion. They add a few drops of dilute hydrochloric acid to acidify the solution, then add aqueous silver nitrate. A white precipitate forms, and the student concludes that the original solution contains chloride ions.

   Explain why this conclusion is not justified.

   1. A It is not justified, because aqueous silver nitrate reacts with a halide ion only in neutral solution; acidifying the sample switches that reaction off, so no precipitate should have formed with acid present.
   2. B It is not justified, because dilute hydrochloric acid dissolves silver nitrate completely without forming any solid, so any precipitate that did appear must have come from a reaction with a halide ion and not from the acid itself.
   3. C It is not justified, because any white precipitate formed with silver nitrate is the standard test result for a carbonate ion rather than a halide, regardless of which acid was used to acidify the sample beforehand.
   4. D It is not justified, because dilute hydrochloric acid itself supplies chloride ions, which would still react with silver nitrate to give a white precipitate, whether or not the original solution contained a halide ion.
4. 41 mark

   A student tests a solution for sulfate ions by adding aqueous barium chloride directly, without first adding any dilute acid. A white precipitate forms, and the student concludes the solution contains sulfate ions.

   Why is this procedure unreliable, and how should it be corrected?

   1. A It is unreliable because barium chloride does not react with sulfate ions unless the solution is first acidified with dilute nitric acid, so a precipitate forming without that step must come from some other ion.
   2. B It is unreliable because a carbonate ion, if present, also reacts with barium chloride to give a white precipitate; dilute hydrochloric acid should be added first to remove any carbonate before the barium chloride is added.
   3. C It is unreliable because aqueous barium chloride is only sparingly soluble and turns visibly cloudy on standing for a few minutes even with no other ions present, so a precipitate would appear in a blank test using pure water alone.
   4. D It is unreliable because sulfate ions give a precipitate with barium chloride only when the mixture is first exposed to ultraviolet light; without that light source, as in an ordinary school test, no precipitate should form.
5. 51 mark

   A solution containing one of Al3+, Ca2+ or Mg2+ is tested by adding aqueous sodium hydroxide, and a white precipitate forms.

   What can be concluded from this observation alone?

   1. A The solution definitely contains Ca2+, because calcium compounds are the ones most commonly tested for among these three ions, making calcium the safest and most likely conclusion to draw from this result.
   2. B The solution contains one of Al3+, Ca2+ or Mg2+, but this test alone cannot distinguish which one is present, since all three of these ions give the same white precipitate with sodium hydroxide, and no other clue is given.
   3. C The solution definitely contains Al3+, because aluminium hydroxide is the only one of these three precipitates that is genuinely white; calcium and magnesium hydroxide instead form paler precipitates.
   4. D The white precipitate shows the solution instead contains Cu2+, because sodium hydroxide is well known for giving a precipitate with copper compounds, and no other explanation for a white precipitate could reasonably apply here.
6. 61 mark

   A white solid is suspected to be one of calcium carbonate, calcium sulfate, potassium carbonate or potassium sulfate. A sample is held in a blue Bunsen flame and burns with a red-orange colour. A separate sample is treated with dilute hydrochloric acid: effervescence occurs, and the gas produced turns limewater cloudy.

   Which compound is the solid?

   1. A Calcium sulfate, because the red-orange flame confirms calcium is present, and any white solid that effervesces with dilute hydrochloric acid must, by definition, be a sulfate rather than a carbonate.
   2. B Potassium carbonate, because the effervescence with dilute acid, producing a gas that turns limewater cloudy, confirms a carbonate is present, and potassium is well known for giving a red-orange flame test result.
   3. C Calcium carbonate, because the red-orange flame test result confirms that calcium is present, and the effervescence, producing a gas that turns limewater cloudy, confirms that a carbonate ion is present too.
   4. D Potassium sulfate, because sulfates are the most common type of compound to give a strongly coloured flame test, and potassium is one of the metals most frequently tested for in this way.
7. 71 mark

   A student heats blue hydrated copper(II) sulfate crystals strongly in a test tube. The crystals turn white, and a colourless liquid collects on the cooler part of the tube.

   Which further test would best confirm that this colourless liquid is water?

   1. A Add a few drops of the liquid to a fresh sample of white anhydrous copper(II) sulfate; if the white solid turns blue, this confirms the liquid contains water.
   2. B Hold a lit splint at the mouth of the test tube containing the liquid, close to its open end; if a characteristic squeaky pop is heard, this confirms the liquid is water rather than any other substance.
   3. C Bubble the collected liquid through a sample of limewater; if the limewater turns cloudy in response, this confirms that the liquid collected from the tube is water.
   4. D Heat the liquid strongly in an open container and observe whether it turns blue as it is heated; if a blue colour appears, this confirms the liquid is water.
8. 81 mark

   A 500 cm^3 sample of dry air is first passed through excess aqueous sodium hydroxide, which removes the carbon dioxide, and then passed over excess heated copper, which removes the oxygen (forming copper oxide). Dry air is approximately 78% nitrogen and 21% oxygen by volume, with the remainder mostly argon and a small amount of carbon dioxide.

   Approximately what volume of gas remains after both steps?

   1. A About 105 cm^3, since heated copper actually removes the nitrogen rather than the oxygen, and removing the nitrogen (with the negligible carbon dioxide) leaves mostly the original oxygen behind.
   2. B About 250 cm^3, since nitrogen and oxygen are assumed here to make up roughly equal shares of dry air by volume, so removing both the carbon dioxide and the oxygen still leaves half the sample.
   3. C About 495 cm^3, since only a very small volume of gas, the carbon dioxide, is ever removed by either of these two steps, with the oxygen and nitrogen both passing through unaffected.
   4. D About 390 cm^3, since removing the oxygen (and the negligible carbon dioxide) leaves mostly the original nitrogen, together with the argon that was already present in the sample of air.
9. 91 mark

   Chlorofluorocarbons (CFCs), once widely used as refrigerants and aerosol propellants, are largely unreactive in the lower atmosphere but are broken down by ultraviolet radiation once they reach the stratosphere, releasing reactive chlorine free radicals.

   Which statement correctly describes the main recognised environmental effect of this?

   1. A The chlorine radicals react with and break down ozone molecules in the stratosphere, thinning the ozone layer over time and allowing more ultraviolet radiation through to the Earth's surface.
   2. B The chlorine radicals combine with ozone molecules to increase the ozone concentration in the stratosphere, thickening the ozone layer and blocking more of the incoming ultraviolet radiation.
   3. C CFCs remain completely unreactive even once they reach the stratosphere, so despite persisting there for a long time, they can have no effect on the ozone layer at all.
   4. D The chlorine radicals released from CFCs absorb infrared radiation directly in the stratosphere, and this direct absorption is the main way that CFCs contribute to warming the Earth's surface.
10. 101 mark

    Which of the following is a recognised effect of increasing atmospheric concentrations of greenhouse gases such as carbon dioxide and methane?

    1. A A rise in average global temperatures, which can cause polar ice and glaciers to melt and sea levels to rise.
    2. B A widening hole in the ozone layer over Antarctica, allowing more ultraviolet radiation to reach the Earth's surface.
    3. C A decrease in the amount of infrared radiation absorbed by the atmosphere, cooling the Earth's surface.
    4. D An increase in the concentration of oxygen in the atmosphere, since photosynthesis increases in a warmer climate.
11. 111 mark

    Nitrogen oxides (NOx) form inside a car engine when nitrogen and oxygen from the air react together at the high temperatures reached during combustion.

    Which statement correctly describes an effect of nitrogen oxides once released into the atmosphere?

    1. A They react with ozone in the upper atmosphere to increase its concentration, improving protection from ultraviolet radiation.
    2. B They contribute to acid rain and can also irritate the respiratory system, causing breathing problems.
    3. C They dissolve in rainwater to form a strongly alkaline solution, which damages plant life.
    4. D They have no significant effect on air quality, since nitrogen and oxygen are both harmless components of clean air.
12. 121 mark

    Some of the carbon dioxide released into the atmosphere by burning fossil fuels dissolves into seawater.

    Which statement correctly describes an effect of this?

    1. A The dissolved carbon dioxide reacts with seawater to form oxygen gas directly, increasing the amount of dissolved oxygen available to marine life living near the surface.
    2. B The dissolved carbon dioxide makes the ocean progressively more alkaline over time, which in turn speeds up the growth of coral reefs and shellfish that build calcium carbonate skeletons.
    3. C The dissolved carbon dioxide has essentially no effect on the chemistry of seawater, since carbon dioxide is not very soluble in water and most of it simply escapes back into the atmosphere.
    4. D The dissolved carbon dioxide forms a weak acid in seawater, making the ocean more acidic and making it harder for marine organisms to build shells and skeletons from calcium carbonate.
13. 131 mark

    A water company adds a small, carefully controlled quantity of fluoride ions to the drinking water supply.

    Which statement best explains why the quantity added must be carefully controlled rather than simply as much as possible?

    1. A Fluoride reduces the incidence of tooth decay at low concentrations, but higher concentrations of fluoride can be harmful, so the dose added must be carefully limited to give a benefit without unwanted side effects.
    2. B Fluoride ions have no known effect on dental or general health at any concentration whatsoever, so the precise amount added to the water supply makes no practical difference either way.
    3. C Fluoride is added to drinking water purely to disinfect it and kill harmful microorganisms, in exactly the same way chlorine is, and any apparent effect on tooth decay is an unconnected coincidence.
    4. D A higher concentration of fluoride in the water supply would always produce a correspondingly greater reduction in tooth decay with no additional health risk, so the amount added should be set as high as is practical.
14. 141 mark

    A white solid is suspected to be either magnesium carbonate or magnesium sulfate. Dilute hydrochloric acid is added to a sample: effervescence occurs, and the gas produced turns limewater cloudy.

    Which conclusion is correctly drawn from this result?

    1. A The solid could be either compound, since both carbonates and sulfates react readily with dilute hydrochloric acid to release a gas that turns limewater cloudy, so the test cannot distinguish between them.
    2. B The solid is magnesium sulfate, since sulfates are well known to react vigorously with dilute hydrochloric acid, releasing carbon dioxide gas that then turns limewater cloudy on contact.
    3. C The solid is magnesium carbonate, since only a carbonate reacts with dilute acid to release a gas that turns limewater cloudy, and both candidate compounds already share the same metal ion.
    4. D No conclusion can be drawn from this result without first carrying out a flame test, since the anion present cannot reliably be identified until the metal ion has been confirmed by some other means.
15. 151 mark

    A student wants to test whether a gas is hydrogen. They insert a glowing splint into a test tube of the gas: the splint does not relight, and no sound is heard. The student concludes that the gas cannot be hydrogen.

    Explain why this conclusion is unsound.

    1. A The conclusion is sound, because a glowing splint is in fact the correct and only recognised test for hydrogen gas, so getting no reaction from it means the gas is definitely not hydrogen.
    2. B The test for hydrogen actually requires a lit, burning splint held at the open end of the tube, not a glowing one; a lit splint produces a characteristic squeaky pop when hydrogen gas is present.
    3. C Hydrogen only produces a squeaky pop with a lit splint if it has first been mixed with an equal volume of carbon dioxide gas, so a pure sample of hydrogen on its own would never react with any splint at all.
    4. D A glowing splint can only be used to test for hydrogen gas if the sample has first been passed through limewater to remove any carbon dioxide that might otherwise interfere with the result.

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: C

   1. Electrolysis of dilute sulfuric acid produces oxygen at the anode (positive electrode) and hydrogen at the cathode (negative electrode).
   2. The test for oxygen is to insert a glowing splint into the gas; if the gas is oxygen, the splint relights.
   3. The gas collected at the anode relights a glowing splint, which matches the oxygen test exactly.
   4. So the gas is oxygen, which is option C.
   • Why not A: Confuses the relighting glowing-splint test for oxygen with the squeaky pop test for hydrogen, which uses a lit splint and produces a small explosion, not a silent relighting.
   • Why not B: Carbon dioxide extinguishes a glowing or lit splint rather than relighting one, so this observation rules out carbon dioxide rather than confirming it.
   • Why not D: Chlorine is identified by its effect on damp blue litmus paper (turning it red, then bleaching it), not by a glowing splint test.

2. Question 2Answer: A

   1. Chlorine dissolves in water to form an acidic solution, so damp blue litmus paper turns red on contact, just as it would with any other acidic gas.
   2. Chlorine's solution also has a bleaching action, so shortly after turning red the litmus paper loses its colour completely and turns white.
   3. Other acidic gases, such as sulfur dioxide, hydrogen chloride and carbon dioxide, turn damp blue litmus paper red but do not then bleach it.
   4. Only chlorine produces both the reddening and the subsequent bleaching, so the gas is chlorine, which is option A.
   • Why not B: Sulfur dioxide dissolves to form an acidic solution that turns blue litmus paper red, but it does not then bleach the paper, so the paper would stay red rather than turning white.
   • Why not C: Hydrogen chloride dissolves to form hydrochloric acid, an acidic solution that turns blue litmus paper red, but it has no bleaching action, so the paper would stay red rather than turning white.
   • Why not D: Carbon dioxide forms only a weakly acidic solution and does not bleach litmus paper at all, so it cannot account for the paper losing all its colour.

3. Question 3Answer: D

   1. Testing for a halide ion requires acidifying with dilute nitric acid before adding aqueous silver nitrate; the acid removes any carbonate ions that would otherwise also give a precipitate.
   2. Hydrochloric acid must not be used for this acidification step, because it itself contains chloride ions.
   3. Those chloride ions from the acid would react with silver nitrate to form a white precipitate regardless of whether the original solution contained any halide ion, giving a false positive result.
   4. So the student's conclusion is unjustified: the white precipitate could simply be silver chloride formed from the acid itself. Nitric acid, which contains no halide ions, avoids this problem, which is option D.
   • Why not A: Invents a false rule that silver nitrate needs a neutral solution to react with a halide ion; acidifying with dilute nitric acid is in fact standard procedure, and the reaction proceeds normally in acidic conditions.
   • Why not B: Wrongly claims hydrochloric acid produces no solid with silver nitrate; in fact the acid's own chloride ions react with the silver nitrate on their own to give a white precipitate of silver chloride, which is exactly why a precipitate can appear even when the original solution contains no halide ion.
   • Why not C: Confuses the anion tests: a white precipitate with silver nitrate identifies a halide (chloride, bromide or iodide, distinguished by colour), while a carbonate ion is tested for separately with dilute acid, not silver nitrate.

4. Question 4Answer: B

   1. Aqueous barium chloride reacts with sulfate ions to form a white precipitate of barium sulfate, which is the basis of the sulfate test.
   2. However, a carbonate ion would also react with barium chloride to give a white precipitate (barium carbonate), so a positive result alone cannot distinguish sulfate from carbonate.
   3. Adding dilute hydrochloric acid before the barium chloride reacts away any carbonate ions present, so any white precipitate that still forms afterwards must be due to sulfate.
   4. The student's test, without this acid step, cannot rule out carbonate as the source of the precipitate, so the correct procedure is to add dilute hydrochloric acid first, which is option B.
   • Why not A: Invents an incorrect requirement: barium chloride reacts directly with sulfate ions with no acid needed, so that part of the test is correct as originally performed; the missing step is ruling out carbonate, not making the reaction happen at all.
   • Why not C: Invents an instability of barium chloride solution that does not occur; the real issue is a different ion (carbonate) giving a false positive, not the reagent clouding on its own.
   • Why not D: Invents a light-dependent requirement that plays no part in this test; sulfate reacts with barium chloride in ordinary daylight, or in the dark, exactly the same way.

5. Question 5Answer: B

   1. Adding aqueous sodium hydroxide to a solution of Al3+, Ca2+ or Mg2+ produces a white precipitate of the corresponding metal hydroxide in each case.
   2. Because all three ions give the same white precipitate, this single observation cannot distinguish between them.
   3. All that can be concluded is that the solution contains one of these three ions, not which specific one, which is option B.
   • Why not A: Assumes calcium without justification; nothing about how commonly an ion is tested favours it as the answer, and this observation alone cannot distinguish calcium from aluminium or magnesium.
   • Why not C: Wrongly claims calcium and magnesium hydroxide are not white; all three metal hydroxides (aluminium, calcium and magnesium) are described as white precipitates, so a white precipitate cannot identify aluminium specifically.
   • Why not D: Confuses the result with the copper test; Cu2+ gives a distinctive blue precipitate with sodium hydroxide, not a white one, so a white precipitate rules out copper rather than confirming it.

6. Question 6Answer: C

   1. A red-orange flame is the characteristic flame test colour for calcium ions.
   2. Effervescence with dilute acid, producing a gas that turns limewater cloudy, is the characteristic test for a carbonate ion.
   3. Matching both observations to their respective ions gives calcium (from the flame test) and carbonate (from the acid test), so the compound is calcium carbonate.
   4. This is option C.
   • Why not A: Correctly identifies calcium from the flame colour but invents a false rule that any acid-reacting white solid must be a sulfate; effervescence producing a gas that turns limewater cloudy is specifically the carbonate test, and calcium sulfate does not react with dilute acid in this way.
   • Why not B: Correctly identifies a carbonate from the acid test but wrongly assigns potassium's flame colour; potassium produces a lilac flame, not red-orange, so the flame observation points to calcium instead.
   • Why not D: Invents an unsupported generalisation about sulfates and flame colours; sulfates do not characteristically give flame colours at all, and potassium in any case gives a lilac flame, not red-orange, matching neither observation.

7. Question 7Answer: A

   1. Heating hydrated copper(II) sulfate drives off its water of crystallisation, turning the blue solid white and producing a colourless liquid.
   2. The test for water is to add the liquid to white anhydrous copper(II) sulfate: if water is present, the white solid turns blue.
   3. Applying a few drops of the collected liquid to a fresh sample of anhydrous copper(II) sulfate and observing a colour change to blue confirms the liquid is water.
   4. This is option A.
   • Why not B: Confuses this with the hydrogen gas test; holding a lit splint to hear a squeaky pop tests for hydrogen gas, and produces no useful information about whether a liquid is water.
   • Why not C: Confuses this with the carbon dioxide gas test; bubbling a gas through limewater to see if it turns cloudy tests for carbon dioxide gas, not for identifying a liquid as water.
   • Why not D: Misapplies the anhydrous copper(II) sulfate test to the liquid itself rather than to the solid; it is the white anhydrous copper(II) sulfate that turns blue in the presence of water, and heating a liquid does not itself turn it blue.

8. Question 8Answer: D

   1. Sodium hydroxide removes the carbon dioxide, which is only a small fraction of dry air and can be treated as negligible for this calculation.
   2. Heated copper reacts with oxygen (2Cu + O2 -> 2CuO), removing it from the sample; oxygen makes up about 21% of dry air, so about 500 x 0.21 = 105 cm^3 is removed.
   3. What remains is mostly the nitrogen, which makes up about 78% of dry air: 500 x 0.78 = 390 cm^3.
   4. So about 390 cm^3 of gas remains, which is option D.
   • Why not A: Swaps which gas is removed by heated copper: heated copper reacts with and removes oxygen, not nitrogen, so nitrogen remains in the sample rather than oxygen.
   • Why not B: Assumes nitrogen and oxygen make up equal shares of dry air, but dry air is about 78% nitrogen and only about 21% oxygen, not a 50:50 split.
   • Why not C: Forgets that heated copper removes the oxygen, which is about 21% of the sample (roughly 105 cm^3), treating only the small carbon dioxide volume as having been removed.

9. Question 9Answer: A

   1. CFCs are largely unreactive in the lower atmosphere, which lets them persist long enough to drift up into the stratosphere.
   2. There, ultraviolet radiation breaks the CFC molecules down, releasing reactive chlorine free radicals.
   3. These chlorine radicals react with and break down ozone molecules; because each radical can go on to react repeatedly, a small quantity of CFCs can destroy a large amount of ozone, thinning the ozone layer and letting more ultraviolet radiation reach the Earth's surface.
   4. This is a different phenomenon from the greenhouse effect caused by gases such as carbon dioxide and methane, so the correct effect is ozone depletion by chlorine radicals, which is option A.
   • Why not B: Reverses the actual chemistry: chlorine radicals catalytically break down ozone molecules and thin the ozone layer, rather than combining with ozone to increase its concentration.
   • Why not C: Contradicts the premise given in the question: CFCs are described as being broken down by ultraviolet radiation once they reach the stratosphere, which is exactly where the reactive chlorine radicals that attack ozone come from.
   • Why not D: Confuses ozone depletion with the greenhouse effect; the recognised effect of CFCs on the ozone layer works through chlorine radicals breaking down ozone molecules, not through directly absorbing infrared radiation, which is the mechanism associated with greenhouse gases such as carbon dioxide and methane.

10. Question 10Answer: A

    1. Increasing concentrations of greenhouse gases such as carbon dioxide and methane enhance the greenhouse effect, trapping more energy in the atmosphere.
    2. A well-established consequence of this is a rise in average global temperatures.
    3. This warming causes further effects such as melting polar ice and glaciers and rising sea levels.
    4. This is a different phenomenon from ozone depletion, which is caused by different substances (CFCs) acting on stratospheric ozone, so option A is correct and option B describes an unrelated process.
    • Why not B: Confuses the enhanced greenhouse effect with ozone depletion; the ozone hole is caused mainly by chlorofluorocarbons (CFCs) reacting with ozone in the stratosphere, a separate phenomenon from the warming caused by greenhouse gases.
    • Why not C: Reverses the actual mechanism; greenhouse gases increase the amount of infrared radiation absorbed and re-radiated by the atmosphere, warming rather than cooling the surface.
    • Why not D: Invents an unsupported chain of reasoning; there is no recognised effect of warming that substantially raises atmospheric oxygen concentration in this way.

11. Question 11Answer: B

    1. Nitrogen oxides released into the atmosphere dissolve in atmospheric moisture to form acids, contributing to acid rain.
    2. They also irritate the respiratory system and can worsen conditions such as asthma.
    3. Although nitrogen oxides are formed from the harmless gases nitrogen and oxygen, the compound they form is itself a harmful pollutant.
    4. So option B, describing acid rain and respiratory irritation, is correct.
    • Why not A: Invents a beneficial effect that does not occur; nitrogen oxides are linked to harmful effects such as acid rain and smog formation, not to increasing protective ozone.
    • Why not C: Reverses the chemistry; nitrogen oxides dissolve in rainwater to form acids (contributing to acid rain), not an alkaline solution.
    • Why not D: Wrongly reasons from the harmless starting elements (nitrogen and oxygen) to conclude the compound they form is also harmless; NOx is a genuinely harmful pollutant despite being formed from two harmless gases.

12. Question 12Answer: D

    1. Carbon dioxide dissolves in seawater and reacts with it to form a weak acid (carbonic acid).
    2. This makes the ocean more acidic over time, a process known as ocean acidification.
    3. Increased acidity makes it more difficult for marine organisms, such as corals and shellfish, to build and maintain shells and skeletons made of calcium carbonate.
    4. So the correct effect is increased ocean acidity harming shell- and skeleton-building organisms, which is option D.
    • Why not A: Invents a reaction that does not occur; dissolving carbon dioxide in seawater does not produce oxygen gas by any known process.
    • Why not B: Reverses the direction of the effect; dissolved carbon dioxide makes the ocean more acidic, not more alkaline, which makes shell and skeleton formation harder rather than easier for these organisms.
    • Why not C: Wrongly claims carbon dioxide is not very soluble in water; carbon dioxide in fact dissolves readily enough in seawater to measurably lower its pH over time, which is the whole basis of ocean acidification.

13. Question 13Answer: A

    1. Fluoride ions are added to drinking water because, at low concentrations, they help to reduce the incidence of tooth decay.
    2. However, fluoride can be harmful at higher concentrations, so simply adding as much as possible is not appropriate.
    3. The quantity added is therefore carefully controlled to give the dental health benefit while avoiding the risks of excess fluoride.
    4. This is option A.
    • Why not B: Denies fluoride's known effect on dental health; fluoride ions are added specifically because they help reduce tooth decay at appropriate, carefully controlled concentrations.
    • Why not C: Confuses fluoride's purpose with chlorine's; chlorine is the disinfectant added to kill harmful microorganisms in the water supply, while fluoride's purpose is specifically to help reduce tooth decay.
    • Why not D: Wrongly assumes there is no upper limit on benefit or risk; too much fluoride is harmful to health, which is exactly why the dose must be carefully controlled rather than maximised.

14. Question 14Answer: C

    1. Effervescence with dilute acid, producing a gas that turns limewater cloudy, is the specific test for a carbonate ion; sulfates do not give this result with dilute acid.
    2. So this observation is conclusive: the anion present is a carbonate, ruling out sulfate.
    3. Because magnesium carbonate and magnesium sulfate share the same metal ion, identifying the anion is all that is needed to identify the compound; no further test, such as a flame test, is required.
    4. So the correct conclusion is option C: the solid is magnesium carbonate.
    • Why not A: Wrongly claims ambiguity; sulfates do not effervesce with dilute acid to release a gas that turns limewater cloudy, so this observation is conclusive for a carbonate and does not also fit a sulfate.
    • Why not B: Invents a reaction that does not occur; sulfates do not react with dilute hydrochloric acid to release carbon dioxide or any other gas, so this observation cannot indicate a sulfate.
    • Why not D: Reverses the logic of chemical testing: identifying an anion by its reaction with dilute acid does not depend on first knowing the metal ion, and here the metal ion is already given as magnesium in both candidate compounds anyway, so a flame test could add nothing.

15. Question 15Answer: B

    1. The test for hydrogen uses a lit splint (one that is burning), not a glowing splint; a glowing splint is instead the test for oxygen, which it relights.
    2. If hydrogen is present, a lit splint held at the open end of the tube causes a small explosion, heard as a 'squeaky pop'.
    3. Because the student used a glowing splint rather than a lit one, this experiment did not actually carry out the correct test for hydrogen, so no valid conclusion about hydrogen can be drawn from it.
    4. So the student's conclusion is unsound, which is option B.
    • Why not A: Affirms the student's flawed reasoning; a glowing splint is actually the test for oxygen (which it relights), not for hydrogen, so this option repeats the same underlying confusion about which test is which.
    • Why not C: Invents an untrue dependency on carbon dioxide; hydrogen gives a squeaky pop with a lit splint entirely on its own, with no need for any other gas to be mixed in first.
    • Why not D: Invents an irrelevant limewater step; limewater is used to test for carbon dioxide gas and has no role at all in testing for hydrogen.