OCR GCSE Chemistry A: Identifying the products of chemical reactions (J248) – Practice Questions

These are original questions written for Marlbridge, for revision and practice on this content. They are not reproduced past-paper questions, and they do not replicate the exam’s exact structure, question count or mark tariffs – examination boards hold copyright in their own papers. Use these alongside the official past papers from your board or school.

Flame, gas, ion and instrument questions follow, all on sub-topic C4.2 Identifying the products of chemical reactions: statements C4.2a to C4.2g, CM4.2i and the PAG C5 practical suggestions in OCR J248, Chemistry A (Gateway Science) at GCSE (9-1), specification Version 4.0 (August 2026), first assessed in 2018. Questions that rely on statement C4.2b, the sodium hydroxide and anion tests, are labelled Higher Tier only; the others suit both tiers. The specification assesses Topic C4 in Paper 2 (Foundation) and Paper 4 (Higher).

Learn the tests first in the C4.2 study guide and the C4.2 revision notes. To see where this fits, open the course page for J248 or the statement checklist. A free diagnostic helps you choose what to practise next.

Questions

1. For each gas, describe the test and give the positive result.

(a) Oxygen [2] (b) Chlorine [2]

2. A student is given three solids to flame test.

(a) Describe how the student should carry out a flame test on one solid. [3] (b) Explain why the wire loop must be cleaned between samples. [1]

3. Four solids give these flame colours: W crimson, X green, Y lilac, Z yellow.

(a) Identify the metal ion in each solid. [4] (b) A fifth solid gives an orange-red flame. Identify its metal ion. [1]

4. (Higher Tier only) A few drops of sodium hydroxide solution, and then an excess, are added to four solutions.

Solution	Few drops	Excess
A	Blue precipitate	No change
B	White precipitate	Precipitate dissolves
C	Orange-brown precipitate	No change
D	White precipitate	No change

(a) Identify the cation in each solution. [4] (b) Give the ionic equation for the precipitate forming in solution C. Include state symbols. [2]

5. (Higher Tier only) A student thinks a solution contains bromide ions.

(a) Name the two reagents the student should add, in order. [2] (b) State the result if bromide ions are present. [1] (c) Explain why the student must not use hydrochloric acid in this test. [1]

6. (Higher Tier only) Two white solids are sodium carbonate and sodium sulfate. Both give a yellow flame. Describe tests, with results, that would show which solid is which. [4]

7. A colour-change test kit can detect a metal ion in water down to 0.5 mg/dm³. A laboratory instrument can detect the same ion down to 0.002 mg/dm³.

(a) Calculate how many times more sensitive the instrument is than the kit. [2] (b) A water sample contains 0.010 mg/dm³ of the ion. State which method(s) would detect it. [1] (c) Give one advantage of the instrument other than sensitivity, and say what it means. [1]

8. A forensic chemist has a small sample of a colourless liquid. Its mass spectrum is shown below as bar heights. Simplified reference data are supplied (use these values).

m/z	Unknown (relative abundance)	Bar
27	15	█▌
43	18	█▊
45	100	██████████
60	5	▌

Reference	Main peaks, m/z (relative abundance)
Propan-1-ol, CH₃CH₂CH₂OH	31 (100), 29 (15), 42 (15), 59 (10), 60 (6)
Propan-2-ol, CH₃CH(OH)CH₃	45 (100), 43 (18), 27 (15), 60 (5)
Ethanoic acid, CH₃COOH	43 (100), 45 (85), 60 (75)

(a) State the relative molecular mass of the unknown and explain how you got it from the chart. [2] (b) Identify the unknown. Use the reference data to justify your answer. [3] (c) Suggest two reasons why the chemist uses an instrument rather than test-tube reactions for this sample. [2]

9. (Higher Tier only) Two unknown solids, X and Y, are each a single ionic compound.

• X: crimson flame. Its solution gives no precipitate with sodium hydroxide. With dilute acid it fizzes, and the gas turns limewater milky.
• Y: orange-red flame. Its solution gives a white precipitate with sodium hydroxide that stays in excess, and a cream precipitate with dilute nitric acid then silver nitrate solution.

(a) Name compound X and write its formula. [3] (b) Name compound Y and write its formula. [3] (c) Write the ionic equation for the cream precipitate forming. [1]

10. (Higher Tier only) Four bottles of solution have lost their labels. They hold iron(II) chloride, iron(III) sulfate, zinc sulfate and calcium chloride. Describe a plan of tests, with the expected result for each solution, that would identify all four. [8]

11. A student adds a catalyst to hydrogen peroxide solution in an open flask on a balance. A gas bubbles off and relights a glowing splint. The balance reads 85.20 g at the start and 84.54 g at the end.

(a) Calculate the change in mass. [1] (b) The student writes: “Mass was destroyed in the reaction.” Explain why this is wrong. [2] (c) Name the gas. [1]

Answers

1. (a) Put a glowing splint into the gas [1]; it relights [1]. (b) Damp blue litmus paper is placed in the gas [1]; it goes red, then the colour is bleached to white [1]. Examiner insight: Give both the test and the result for each gas; “relights” with a lighted splint, or bleaching with dry paper, is unlikely to gain the marks.

2. (a) Clean a nichrome loop by dipping it in hydrochloric acid and heating it until it gives no colour [1]. Dip the loop in acid and then in the solid [1]. Hold it at the edge of a blue (roaring) Bunsen flame and record the colour [1]. (b) Traces of the previous solid would give their own colour, giving a false result [1]. Examiner insight: Say “blue flame”; an answer that just says “put it in the flame” leaves out the condition that makes the colour visible.

3. (a) W: lithium, Li⁺ [1]. X: copper, Cu²⁺ [1]. Y: potassium, K⁺ [1]. Z: sodium, Na⁺ [1]. (b) Calcium, Ca²⁺ [1]. Examiner insight: Crimson and orange-red are easily swapped; tie crimson to lithium and orange-red to calcium, and do not write a vague “red”.

4. (a) A: Cu²⁺ [1]. B: Zn²⁺ [1]. C: Fe³⁺ [1]. D: Ca²⁺ [1]. (b) Fe³⁺ + 3OH⁻ → Fe(OH)₃ with the correct balancing [1]; Fe³⁺(aq) + 3OH⁻(aq) → Fe(OH)₃(s) with all state symbols correct [1]. Examiner insight: Write “iron(III)” or Fe³⁺, not just “iron”; the oxidation state is what separates C from an iron(II) solution.

5. (a) Dilute nitric acid [1], then silver nitrate solution [1]. (b) A cream precipitate (silver bromide) [1]. (c) The chloride ions in hydrochloric acid react with silver nitrate too, so a white precipitate would appear even with no halide in the sample [1]. Examiner insight: The order matters: acid first, then silver nitrate. Naming the reagents in the wrong order, or “acid” with no name, weakens the answer.

6. Add dilute hydrochloric acid to each solid [1]; sodium carbonate fizzes and the gas turns limewater milky [1]. Dissolve the other solid and add barium chloride solution that has been acidified with dilute hydrochloric acid [1]; a white precipitate shows sodium sulfate [1]. Examiner insight: The flame test cannot separate these two, so marks come from the anion tests; give a reagent and an observation for each.

7. (a) 0.5 ÷ 0.002 [1] = 250 times more sensitive [1]. (b) Only the instrument: 0.010 mg/dm³ is above the instrument’s limit but below the kit’s limit [1]. (c) Speed: results come quickly and many samples can be run; or accuracy: the result is reliable and close to the true value [1]. Examiner insight: For “how many times”, divide the larger limit by the smaller and give a plain number; a lower detection limit means a more sensitive method.

8. (a) The peak with the highest m/z is at 60 [1], so Mr = 60 [1]. (b) All three references have Mr 60, so the fragment pattern decides [1]. The base peak of the unknown is at 45, which rules out propan-1-ol (base peak 31) [1]. Ethanoic acid has a large peak at 60 (75) and its base peak at 43, but the unknown’s peaks at 27, 43, 45 and 60 match propan-2-ol in position and height, so it is propan-2-ol [1]. (c) First reason, for example sensitivity: a very small forensic sample is enough [1]. Second reason, for example accuracy (the identification is reliable) or speed (results come quickly) [1]. Examiner insight: A justification that only says “it matches” gains less than one that names the matching peaks and the peak that rules out each other reference.

9. (a) Crimson flame: lithium ions [1]; fizzing with limewater milky: carbonate ions [1]; lithium carbonate, Li₂CO₃ [1]. (b) Orange-red flame and white hydroxide that stays in excess: calcium ions [1]; cream precipitate with acidified silver nitrate: bromide ions [1]; calcium bromide, CaBr₂ [1]. (c) Ag⁺(aq) + Br⁻(aq) → AgBr(s) [1]. Examiner insight: Show which result points to which ion; a bare name with no reasoning gives the marker nothing to credit if the final formula slips.

10. Put a fresh sample of each solution in a separate test tube and add a few drops of sodium hydroxide solution [1].

• Green precipitate: iron(II) chloride [1].
• Orange-brown precipitate: iron(III) sulfate [1].
• The other two give white precipitates. Add excess sodium hydroxide [1].
• The white precipitate that dissolves in excess is zinc sulfate [1]; the one that stays is calcium chloride [1].

Confirm the anions on fresh samples. Dilute hydrochloric acid then barium chloride gives a white precipitate with iron(III) sulfate and zinc sulfate [1]. Dilute nitric acid then silver nitrate gives a white precipitate with iron(II) chloride and calcium chloride [1]. Examiner insight: In a long plan, set the tests out in a clear order; state the reagent, the observation and the conclusion for every bottle, and use fresh samples for each test.

11. (a) 85.20 − 84.54 = 0.66 g decrease [1]. (b) Mass is conserved: atoms are not created or destroyed [1]. The flask is open, so the gas produced escapes into the air, and its mass is no longer on the balance [1]. (c) Oxygen [1]. Examiner insight: Link the mass change to the gas leaving an open container; repeating “mass is conserved” without saying where the mass went is unlikely to earn both marks.

Where marks are usually lost

• Naming a test without its result, or a result without the test.
• Mixing up the splints: glowing is the oxygen test, lighted is the hydrogen test.
• Writing “red” for both lithium and calcium flames.
• Stopping after “white precipitate” without adding excess sodium hydroxide to separate zinc from calcium.
• Writing “iron” instead of iron(II) or iron(III).
• Adding hydrochloric acid before silver nitrate, or sulfuric acid before barium chloride.
• Reading Mr from the tallest peak instead of the highest m/z peak.
• Justifying a spectrum match with “it looks the same” instead of naming peaks.
• Giving “more reliable” or “better” as an instrumental advantage without using sensitivity, accuracy or speed.

Next steps

• Go back over the test tables in the C4.2 revision notes.
• For any question you found hard, read that part of the C4.2 study guide.
• Book a free trial class.
• Other J248 topics are listed on the course page.
• Mark C4.2 complete in your J248 checklist.
• Pick one of the free 10-minute diagnostics to find your next topic.

Official syllabus

Written for sub-topic C4.2 Identifying the products of chemical reactions. Document: the J248 specification from OCR, Chemistry A (Gateway Science) for GCSE (9-1), Version 4.0 of August 2026, first assessment 2018.