📋 Structured Questions
These questions test key concepts from Topic 8. Attempt each question on paper, then click "Show Mark Scheme" to check your answer.
Question 1: Testing for Chlorine Gas
2 marksChlorine is used to treat some ground water.
(a) Describe the test for chlorine gas. Give the result of the test. [2]
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- Test: use damp litmus paper [1]
- Result: the paper is bleached / turns white [1]
Question 2: Chromatography Apparatus Errors
2 marksA student investigated an orange food colouring using paper chromatography. The diagram shows the apparatus set up by the student. The student made two mistakes when setting up the apparatus.
(a) Give two mistakes the student made. [2]
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- The start line is drawn in ink (should be pencil — ink would dissolve and run) [1]
- The start line is below the water/solvent level (should be above — otherwise the spots dissolve into the solvent) [1]
Question 3: Calculating Rf Values
3 marksA student investigated green ink using paper chromatography. The Rf value of the yellow dye = 0.60. The distance moved by the yellow dye = 5.7 cm.
(a) Calculate the distance moved by the solvent. [3]
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- 0.60 = 5.7 ÷ distance moved by solvent [1]
- distance = 5.7 ÷ 0.60 [1]
- = 9.5 cm [1]
Question 4: Testing for Ions
3 marksA student tested copper sulfate solution and calcium iodide solution. The student added sodium hydroxide solution to both solutions.
(a) Give the results of the tests. [2]
(b) To test for sulfate ions, the student added dilute hydrochloric acid to the copper sulfate solution. Name the solution that would show the presence of sulfate ions when added to this mixture. [1]
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- Copper sulfate: blue precipitate [1]
- Calcium iodide: white precipitate [1]
- Barium chloride solution (or barium nitrate solution) [1]
Question 5: Pure Substances & Everyday Purity
3 marksGround water often requires treatment before it is safe to drink. After treatment, a company sells the ground water as "pure water." However, the water is not chemically pure because it contains dissolved sodium ions and chloride ions.
(a) Explain why the ground water requires sterilisation before it is safe to drink. [2]
(b) Suggest what the company means by "pure." [1]
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- The ground water contains microbes which are harmful (to health) [1]
- The water is sterilised / the microbes are destroyed [1]
- Company's meaning: unadulterated / in its natural state / nothing has been added to it [1]
Question 6: Flame Tests & Emission Spectroscopy
3 marksSome fireworks contain a mixture of metal ions. Flame emission spectroscopy is used to identify the metal ions.
(a) Why is it difficult to identify the metal ions from the colour of the flame? [1]
(b) Two metal ions were identified in a firework sample using flame emission spectroscopy. Their spectra matched lithium and sodium. Name the two metal ions. [2]
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- The flame colours are masked / mix / blend / only see one colour [1]
- Li⁺ / Lithium [1]
- Na⁺ / Sodium [1]
Question 7: Identifying an Unknown Compound ⭐ Extended Response
6 marksA student is given an unknown white solid. They suspect the solid is potassium bromide.
(a) Describe the chemical tests the student should perform to prove the solid is potassium bromide, including the positive observations for each test. [6]
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Level 3 (5–6 marks): Valid methods for both cation and anion testing are logically sequenced, and correct positive observations are given for each test.
Level 2 (3–4 marks): Both tests described with some correct observations, but may miss key procedural steps (e.g. adding acid first) or not fully link the test to the specific ion.
Level 1 (1–2 marks): One correct test or positive observation is identified (e.g. just the flame test colour for potassium).
Indicative content:
- Test for K⁺ (cation): flame test — dip clean nichrome wire loop into sample, place in a blue Bunsen flame; lilac flame confirms potassium
- Test for Br⁻ (anion): dissolve the solid in distilled water
- Add a few drops of dilute nitric acid (to remove carbonate impurities)
- Then add a few drops of silver nitrate solution
- A cream precipitate confirms bromide ions
Question 8: Required Practical — Chromatography & Rf Values 🔬
4 marksA student separates the dyes in a food colouring using paper chromatography. The solvent travels 10.0 cm from the baseline. One dye spot travels 6.5 cm from the baseline.
(a) Calculate the Rf value of this dye. [1]
(b) The Rf values of known dyes are: Red 40 = 0.45, Yellow 5 = 0.65, Blue 1 = 0.80. Identify the dye and explain your reasoning. [1]
(c) Explain why the baseline must be drawn in pencil, not ink. [1]
(d) State why the solvent level must be below the pencil baseline. [1]
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- (a) Rf = 6.5 ÷ 10.0 = 0.65 [1]
- (b) The dye is Yellow 5 because its Rf value matches (0.65) [1]
- (c) Pencil is insoluble in the solvent, so it won't dissolve and interfere with the results [1]
- (d) So the spots do not dissolve directly into the solvent / the solvent must travel up through the paper [1]
Question 9: Required Practical — Identifying Unknown Ions ⭐🔬 Extended Response
6 marksA student is given an unknown white crystalline solid. They are told it contains a metal cation and a halide anion.
(a) Describe the tests the student should carry out to identify both the cation and the anion in the unknown solid. Include the expected observations for each test. [6]
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Level 3 (5–6 marks): Detailed descriptions of both the flame test (for the cation) and the silver nitrate test (for the anion), with correct procedures and expected observations clearly stated.
Level 2 (3–4 marks): Both tests described with some correct observations, but may miss key procedural detail (e.g. adding nitric acid before silver nitrate).
Level 1 (1–2 marks): One test described or observations given without a clear method.
Indicative content:
- Cation test (flame test): clean a nichrome wire loop by dipping in HCl and heating until no colour; dip into sample; hold in blue Bunsen flame
- Observe flame colour: lilac = potassium, yellow = sodium, red = lithium, green = barium, crimson = calcium
- Anion test (halide): dissolve the solid in distilled water
- Add a few drops of dilute nitric acid first (removes impurities that could give false precipitates)
- Then add silver nitrate solution
- Precipitate colour: white = chloride, cream = bromide, yellow = iodide