ESAT Chemistry: Chemical analysis and the atmosphere, set 1

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

1. 11 mark

   A student reacts magnesium ribbon with dilute hydrochloric acid and collects the gas produced in a test tube. A lit splint is held at the open end of the tube, and the gas ignites with a distinctive 'squeaky pop'.

   Which gas is present in the test tube?

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

   A colourless gas is bubbled through limewater, which turns cloudy. When a lit splint is held in a separate sample of the same gas, the splint is extinguished rather than relit or ignited.

   Which gas is being tested?

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

   A solution of an unknown potassium halide is tested by first adding dilute nitric acid and then adding aqueous silver nitrate. A cream precipitate forms.

   Which halide ion is present in the solution?

   1. A Chloride
   2. B Bromide
   3. C Iodide
   4. D Sulfate
4. 41 mark

   A student has two unlabelled solutions, one containing a carbonate and one containing a sulfate.

   Which pair of tests would correctly identify the carbonate and the sulfate?

   1. A Dilute acid for the carbonate (effervescence, and the gas produced turns limewater cloudy); aqueous silver nitrate for the sulfate (white precipitate).
   2. B Aqueous barium chloride with dilute hydrochloric acid for the carbonate (white precipitate); dilute acid for the sulfate (effervescence).
   3. C Aqueous sodium hydroxide for the carbonate (white precipitate); dilute acid for the sulfate (effervescence).
   4. D Dilute acid for the carbonate (effervescence, and the gas produced turns limewater cloudy); aqueous barium chloride with dilute hydrochloric acid for the sulfate (white precipitate).
5. 51 mark

   Aqueous sodium hydroxide is added dropwise to a solution of an unknown copper salt. A precipitate forms.

   What colour is the precipitate, and which ion does this confirm is present?

   1. A White precipitate, confirms Ca2+
   2. B Green precipitate, confirms Fe2+
   3. C Blue precipitate, confirms Cu2+
   4. D Brown precipitate, confirms Fe3+
6. 61 mark

   Two unlabelled solutions each contain an iron salt: one is iron(II) sulfate and the other is iron(III) sulfate. Aqueous sodium hydroxide is added to each.

   Which observation correctly distinguishes the two solutions?

   1. A The iron(II) sulfate solution gives a green precipitate, while the iron(III) sulfate solution gives a brown precipitate.
   2. B The iron(II) sulfate solution gives a brown precipitate, while the iron(III) sulfate solution gives a green precipitate.
   3. C Both solutions give the same blue precipitate, so sodium hydroxide cannot distinguish them.
   4. D The iron(II) sulfate solution gives a white precipitate, while the iron(III) sulfate solution gives a green precipitate.
7. 71 mark

   Which row correctly matches each metal to the colour it produces in a flame test?

   1. A Lithium: lilac; Sodium: crimson red; Potassium: yellow-orange.
   2. B Lithium: crimson red; Sodium: yellow-orange; Potassium: lilac.
   3. C Lithium: yellow-orange; Sodium: lilac; Potassium: crimson red.
   4. D Lithium: green; Sodium: red-orange; Potassium: yellow-orange.
8. 81 mark

   An unknown solid salt is held in a blue Bunsen flame on a clean nichrome wire. The flame turns lilac.

   Which metal ion is most likely present in the salt?

   1. A Lithium
   2. B Sodium
   3. C Calcium
   4. D Potassium
9. 91 mark

   A student adds a few drops of a colourless liquid to a sample of white anhydrous copper(II) sulfate powder. The powder turns blue.

   What does this observation show about the liquid?

   1. A The liquid contains water.
   2. B The liquid is pure ethanol, which always turns anhydrous copper(II) sulfate blue.
   3. C The liquid is acidic, since acids turn anhydrous copper(II) sulfate blue.
   4. D The liquid contains dissolved carbon dioxide, since carbon dioxide turns anhydrous copper(II) sulfate blue.
10. 101 mark

    Which row gives the approximate percentage by volume of nitrogen and oxygen in dry air?

    1. A About 21% nitrogen and about 78% oxygen, with the remainder mostly argon and carbon dioxide.
    2. B About 50% nitrogen and about 50% oxygen, with no other significant gases.
    3. C About 78% nitrogen and about 21% oxygen, with the remainder mostly argon and a small amount of carbon dioxide.
    4. D About 96% nitrogen and about 4% oxygen, with the remainder mostly argon and carbon dioxide.
11. 111 mark

    Liquid air is separated into its components by fractional distillation.

    Which statement correctly describes how this separation happens?

    1. A The liquid air is passed over a hot catalyst, which reacts selectively with nitrogen and leaves oxygen and argon behind.
    2. B The liquid air is allowed to warm up slowly, and each component gas boils off and is collected separately at its own boiling point, with nitrogen (the lowest boiling point) leaving first.
    3. C The liquid air is frozen solid, and the different gases are separated by filtering out the solid crystals that form first.
    4. D The liquid air is spun in a centrifuge, and denser gases such as oxygen sink while lighter gases such as nitrogen rise to be collected.
12. 121 mark

    Which row correctly identifies a major human-related source for each of the two greenhouse gases named?

    1. A Carbon dioxide: mainly from cattle and rice paddy fields. Methane: mainly from burning fossil fuels and deforestation.
    2. B Carbon dioxide: mainly from nuclear power stations. Methane: mainly from volcanic eruptions.
    3. C Carbon dioxide: mainly from photosynthesis in plants. Methane: mainly from burning fossil fuels.
    4. D Carbon dioxide: mainly from burning fossil fuels and deforestation. Methane: mainly from sources such as cattle and other livestock, rice paddy fields and decomposing rubbish in landfill sites.
13. 131 mark

    Which statement correctly explains how greenhouse gases such as carbon dioxide and methane contribute to warming the Earth's atmosphere?

    1. A They absorb infrared radiation given out by the Earth's surface and re-radiate some of it back towards the surface, trapping energy that would otherwise escape into space.
    2. B They absorb ultraviolet radiation from the Sun before it reaches the Earth's surface, so more energy is retained in the upper atmosphere.
    3. C They react chemically with oxygen in the atmosphere, and this reaction releases heat directly into the surrounding air.
    4. D They form a physical barrier at ground level that blocks incoming sunlight from reaching the Earth's surface at all.
14. 141 mark

    Which row correctly matches each gaseous pollutant to a harmful effect it causes?

    1. A Carbon monoxide: causes acid rain. Sulfur dioxide: is toxic because it binds to haemoglobin in the blood.
    2. B Carbon monoxide: causes global warming only, with no toxic effect on humans. Sulfur dioxide: has no known harmful effects.
    3. C Carbon monoxide: is toxic because it binds to haemoglobin in the blood, reducing its ability to carry oxygen. Sulfur dioxide: causes acid rain and can trigger respiratory problems.
    4. D Carbon monoxide: causes acid rain by reacting with rainwater to form sulfuric acid. Sulfur dioxide: is toxic because it binds to haemoglobin in the blood.
15. 151 mark

    Which statement correctly describes the purposes of adding chlorine and fluoride ions during the treatment of drinking water?

    1. A Chlorine is added to reduce tooth decay, and fluoride ions are added to kill harmful microorganisms.
    2. B Chlorine is added to kill harmful microorganisms, and fluoride ions are added to help reduce tooth decay.
    3. C Both chlorine and fluoride ions are added purely to improve the taste of the water, with no effect on health.
    4. D Chlorine is added to remove dissolved solids from the water, and fluoride ions are added to kill harmful microorganisms.

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: A

   1. Hydrogen is identified by holding a lit splint at the open end of a test tube containing the gas.
   2. If hydrogen is present, it ignites with a small explosion that makes a distinctive 'squeaky pop' sound.
   3. Magnesium reacting with dilute hydrochloric acid produces hydrogen gas (Mg + 2HCl -> MgCl2 + H2), which matches the pop test described.
   4. So the gas is hydrogen, which is option A.
   • Why not B: Confuses the squeaky pop test for hydrogen with the glowing-splint relighting test used to identify oxygen.
   • Why not C: Confuses the squeaky pop test with the limewater test, which is how carbon dioxide is identified, not hydrogen.
   • Why not D: Confuses the squeaky pop test with the bleaching of damp litmus paper, which is how chlorine is identified, not hydrogen.

2. Question 2Answer: C

   1. Carbon dioxide is identified using the limewater test: bubbling the gas through limewater (aqueous calcium hydroxide) turns it cloudy because insoluble calcium carbonate forms.
   2. Carbon dioxide also does not support combustion, so a lit splint held in it goes out, unlike oxygen (which relights a glowing splint) or hydrogen (which pops).
   3. Both observations, cloudy limewater and an extinguished splint, point to carbon dioxide, which is option C.
   • Why not A: Assumes any gas that gives a clear splint result must be oxygen, but oxygen relights a glowing splint rather than extinguishing a lit one, and does not turn limewater cloudy.
   • Why not B: Confuses the limewater test (for carbon dioxide) with the squeaky pop test (for hydrogen); hydrogen does not turn limewater cloudy.
   • Why not D: Assumes any positive gas test must indicate chlorine, but chlorine bleaches damp blue litmus paper rather than simply extinguishing a splint, and does not turn limewater cloudy.

3. Question 3Answer: B

   1. Dilute nitric acid is added first to remove any carbonate ions, which could otherwise also give a precipitate with silver nitrate and confuse the result.
   2. Aqueous silver nitrate then reacts with a halide ion to give a silver halide precipitate: chloride gives white, bromide gives cream, and iodide gives yellow.
   3. A cream precipitate therefore identifies bromide ions, which is option B.
   • Why not A: Chloride gives a white precipitate with silver nitrate, not a cream one.
   • Why not C: Iodide gives a yellow precipitate with silver nitrate, not a cream one.
   • Why not D: Sulfate is not a halide and is not detected by the silver nitrate test at all; it is identified separately using barium chloride, which gives a white precipitate.

4. Question 4Answer: D

   1. A carbonate ion is identified by adding a dilute acid: it effervesces (fizzes) as carbon dioxide gas is released, and that gas turns limewater cloudy when bubbled through it.
   2. A sulfate ion is identified by adding aqueous barium chloride in the presence of dilute hydrochloric acid: an insoluble white precipitate of barium sulfate forms.
   3. Matching each ion to its own test gives dilute acid for the carbonate and barium chloride with dilute hydrochloric acid for the sulfate, which is option D.
   • Why not A: Correctly tests the carbonate with dilute acid, but wrongly uses silver nitrate, the halide test, for the sulfate instead of barium chloride with dilute hydrochloric acid.
   • Why not B: Swaps the two tests around: barium chloride with dilute hydrochloric acid is the sulfate test, and dilute acid producing effervescence is the carbonate test, not the other way round.
   • Why not C: Wrongly uses the sodium hydroxide test, which identifies metal cations, for the carbonate anion, and describes a precipitate result that does not occur when sodium hydroxide is added to a carbonate solution.

5. Question 5Answer: C

   1. Adding aqueous sodium hydroxide to a solution of metal ions precipitates the corresponding insoluble metal hydroxide, and the colour of that precipitate identifies the metal ion.
   2. Cu2+ ions form a blue precipitate of copper(II) hydroxide with sodium hydroxide.
   3. So a blue precipitate confirms the presence of Cu2+, which is option C.
   • Why not A: Gives the precipitate colour for Al3+, Ca2+ or Mg2+, not for Cu2+, which gives a blue precipitate.
   • Why not B: Gives the precipitate colour for Fe2+ (green), not for Cu2+, which gives a blue precipitate instead.
   • Why not D: Gives the precipitate colour for Fe3+ (brown), not for Cu2+, which gives a blue precipitate instead.

6. Question 6Answer: A

   1. Sodium hydroxide precipitates each iron ion as its hydroxide, and the two oxidation states give different colours.
   2. Fe2+ (iron(II)) forms a green precipitate of iron(II) hydroxide.
   3. Fe3+ (iron(III)) forms a brown precipitate of iron(III) hydroxide.
   4. So iron(II) sulfate gives a green precipitate and iron(III) sulfate gives a brown precipitate, which is option A.
   • Why not B: Swaps the two precipitate colours around: Fe2+ gives green and Fe3+ gives brown, not the other way round.
   • Why not C: Blue is the precipitate colour for Cu2+, not for either iron ion, and the two iron ions do in fact give different colours with sodium hydroxide.
   • Why not D: White is the precipitate colour for Al3+, Ca2+ or Mg2+, not for Fe2+, which gives a green precipitate instead.

7. Question 7Answer: B

   1. Each metal ion gives a characteristic flame colour: lithium burns crimson red, sodium burns yellow-orange, potassium burns lilac, calcium burns red-orange and copper burns green.
   2. Only option B lists all three of lithium, sodium and potassium against their correct colours: crimson red, yellow-orange and lilac respectively.
   3. So option B is correct.
   • Why not A: Cycles the three colours one position away from their correct metal: lilac belongs to potassium and crimson red to lithium, not to sodium and lithium as shown here.
   • Why not C: Cycles the three colours the other way round: yellow-orange belongs to sodium and crimson red to lithium, not to lithium and potassium as shown here.
   • Why not D: Uses green, which is copper's flame colour, for lithium, and gives sodium a 'red-orange' that is actually calcium's colour, while still misplacing potassium's lilac.

8. Question 8Answer: D

   1. A flame test colour is characteristic of the metal ion present in the salt.
   2. A lilac flame is the characteristic colour for potassium ions.
   3. Lithium, sodium and calcium each give a different colour (crimson red, yellow-orange and red-orange respectively), so none of them match the lilac flame observed.
   4. So the salt most likely contains potassium, which is option D.
   • Why not A: Lithium produces a crimson red flame, not a lilac one.
   • Why not B: Sodium produces a yellow-orange flame, not a lilac one.
   • Why not C: Calcium produces a red-orange flame, not a lilac one.

9. Question 9Answer: A

   1. Anhydrous copper(II) sulfate is white and reacts with water to form hydrated copper(II) sulfate, which is blue.
   2. This colour change from white to blue is used as a specific chemical test for the presence of water in a sample.
   3. So the colour change shows that the liquid contains water, which is option A (the test alone does not prove the liquid is pure water, only that water is present in it).
   • Why not B: Confuses the copper(II) sulfate test, which specifically confirms water, with a property of ethanol; pure, water-free ethanol does not turn anhydrous copper(II) sulfate blue.
   • Why not C: Invents an acidity link; the white-to-blue colour change of anhydrous copper(II) sulfate is a specific test for the presence of water, not for acidity.
   • Why not D: Invents a carbon dioxide link; the white-to-blue colour change of anhydrous copper(II) sulfate is a specific test for the presence of water, and dissolved carbon dioxide is instead tested for using limewater.

10. Question 10Answer: C

    1. Dry air is a mixture of gases whose composition by volume is roughly constant: about 78% nitrogen and about 21% oxygen.
    2. The remaining roughly 1% is mostly argon (a noble gas), with a much smaller amount of carbon dioxide and traces of other gases.
    3. So the row giving about 78% nitrogen and about 21% oxygen, with the remainder mostly argon and a little carbon dioxide, is correct, which is option C.
    • Why not A: Swaps the two main percentages around: dry air is about 78% nitrogen and about 21% oxygen, not the other way round.
    • Why not B: Invents an equal split between the two gases and ignores that nitrogen is by far the larger component of dry air, at around 78% compared with 21% oxygen.
    • Why not D: Understates oxygen far below its real share of about 21%, and overstates nitrogen well above its real share of about 78%.

11. Question 11Answer: B

    1. Fractional distillation of liquid air relies on the different components having different boiling points.
    2. As the liquid air is allowed to warm up gradually, each gas boils off once its own boiling point is reached and can be collected separately.
    3. Nitrogen has the lowest boiling point of the main components, so it boils off first, followed later by argon and then oxygen.
    4. So the correct description is that the liquid warms and each gas boils off and is collected at its own boiling point, which is option B.
    • Why not A: Invents a catalytic reaction step; fractional distillation is a physical separation based on differences in boiling point, not a chemical reaction with a catalyst.
    • Why not C: Invents a freezing and filtering method; the components are already liquid at the start of the process and are separated by boiling off at different temperatures, not by crystallising solids.
    • Why not D: Invents a centrifuge-based separation; fractional distillation separates by boiling point as the liquid warms, not by spinning to separate gases by density.

12. Question 12Answer: D

    1. Carbon dioxide's major human-related sources are the combustion of fossil fuels (coal, oil and gas) and deforestation, which reduces the amount of carbon dioxide removed by photosynthesis.
    2. Methane's major human-related sources include livestock such as cattle (through digestion), rice paddy fields and the decomposition of waste in landfill sites.
    3. So the row that correctly assigns fossil fuels and deforestation to carbon dioxide, and livestock, rice paddies and landfill sites to methane, is option D.
    • Why not A: Swaps the two sources round: cattle and rice paddies are sources of methane, and burning fossil fuels and deforestation are sources of carbon dioxide, not the other way round.
    • Why not B: Invents sources for both gases: nuclear power stations do not burn carbon-based fuel to release carbon dioxide, and volcanic eruptions are not the major human-related source of methane.
    • Why not C: Mistakes photosynthesis, which removes carbon dioxide from the air, for a source of it, and gives methane the wrong (carbon-dioxide) source of burning fossil fuels.

13. Question 13Answer: A

    1. The Earth's surface absorbs energy from the Sun (mostly as visible light) and re-emits some of that energy as infrared radiation.
    2. Greenhouse gases such as carbon dioxide and methane absorb some of this outgoing infrared radiation and re-radiate part of it back towards the surface.
    3. This traps additional energy in the atmosphere that would otherwise escape into space, warming the atmosphere, which is the enhanced greenhouse effect.
    4. So the mechanism described in option A, absorbing and re-radiating infrared radiation from the Earth, is correct.
    • Why not B: Confuses the greenhouse mechanism, absorbing and re-radiating infrared radiation given out by the Earth, with ultraviolet absorption in the upper atmosphere, which is not how the greenhouse effect works.
    • Why not C: Invents a direct exothermic reaction with oxygen; greenhouse gases warm the atmosphere by absorbing and re-radiating infrared radiation, not by reacting chemically to release heat.
    • Why not D: Invents a physical barrier that blocks all incoming sunlight; greenhouse gases let most incoming sunlight through and instead trap outgoing infrared radiation from the warmed surface.

14. Question 14Answer: C

    1. Carbon monoxide is toxic because it binds to haemoglobin in red blood cells more strongly than oxygen does, reducing the blood's ability to carry oxygen around the body.
    2. Sulfur dioxide, released when fuels containing sulfur impurities are burned, dissolves in atmospheric moisture to form acids, contributing to acid rain, and it can also irritate the respiratory system.
    3. So the row that gives carbon monoxide the haemoglobin-binding toxic effect and sulfur dioxide the acid rain and respiratory effect is correct, which is option C.
    • Why not A: Swaps the two effects around: carbon monoxide is the one that binds to haemoglobin, and sulfur dioxide is the one linked to acid rain, not the other way round.
    • Why not B: Denies both gases their real, well-established harmful effects; carbon monoxide is toxic to humans and sulfur dioxide does cause acid rain and respiratory harm.
    • Why not D: Confuses carbon monoxide with sulfur dioxide for the acid rain effect, wrongly naming sulfuric acid as coming from carbon monoxide instead of from sulfur dioxide reacting with water and oxygen in the atmosphere.

15. Question 15Answer: B

    1. Chlorine is added to drinking water during treatment to kill harmful microorganisms such as bacteria, making the water safe to drink.
    2. Fluoride ions are added, in controlled small amounts, because they help to reduce the incidence of tooth decay.
    3. So chlorine's purpose is disinfection and fluoride's purpose is reducing tooth decay, which is option B.
    • Why not A: Swaps the two purposes round: chlorine is the disinfectant that kills microorganisms, and fluoride is the one added for dental health, not the other way round.
    • Why not C: Denies both real purposes; chlorine and fluoride are added for disinfection and dental health respectively, not purely for taste.
    • Why not D: Gives chlorine the wrong purpose, removing dissolved solids, and wrongly assigns fluoride the disinfecting role instead of chlorine.