IGCSE Chemistry: Identification of Ions and Gases — 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 available free from your board.

Related: Identification of Ions and Gases revision notes

Tier note: everything on this page is 0620 Core (syllabus 12.5, which has no Supplement outcomes). 5070 has no tiers, so O Level candidates need all of it too.

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Questions

1. State the flame colour for: lithium, sodium, potassium, calcium, copper(II). [5]

2. Sodium hydroxide solution is added to four unknown solutions.

(a) State the observation for Cu²⁺, Fe²⁺, Fe³⁺ and Zn²⁺. [4] (b) Describe how zinc can be distinguished from aluminium, both of which give a white precipitate. [2] (c) Describe the test for ammonium ions using sodium hydroxide. [3]

3. Describe the test for:

(a) carbonate ions [2] (b) sulfate ions, including why the solution is acidified first [3] (c) chloride ions, including why the solution is acidified first [3]

4. State the test and positive result for:

(a) hydrogen [2] (b) oxygen [2] (c) carbon dioxide [2] (d) chlorine [2] (e) ammonia [2]

5. A white solid dissolves in water. The solution gives a lilac flame, and a white precipitate with acidified barium nitrate. Identify the compound and justify. [3]

6. Describe how you would distinguish between solutions of bromide ions and iodide ions using one test. [3]

7. Describe how you would use aqueous ammonia to distinguish copper(II) ions from chromium(III) ions. [3]

8. Describe the test for nitrate ions, including the gas that confirms a positive result. [3]

9. (a) A colourless gas turns acidified aqueous potassium manganate(VII) from purple to colourless. Identify the gas. [1] (b) Describe the test for sulfite ions in a solution, and give the result. [2]

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10. Describe how a flame test is used to show that a solid contains potassium ions, and give the result of the test. [2]

11. Describe the test for chloride ions in a solution and give the positive result. [2]

Answers

1. Lithium red [1]; sodium yellow [1]; potassium lilac [1]; calcium orange-red [1]; copper(II) blue-green [1].

2. (a) Cu²⁺ — light blue precipitate [1]. Fe²⁺ — green precipitate, turning brown near the surface on standing [1]. Fe³⁺ — red-brown precipitate [1]. Zn²⁺ — white precipitate [1]. (b) Add aqueous ammonia, then excess ammonia [1]. Both give a white precipitate at first; zinc hydroxide dissolves in excess ammonia to give a colourless solution, while aluminium hydroxide does not dissolve [1]. (Excess sodium hydroxide does not tell them apart: both precipitates dissolve in it.) (c) Add sodium hydroxide and warm [1]. Ammonia gas is released [1], turning damp red litmus paper blue [1].

3. (a) Add dilute hydrochloric acid [1]; effervescence occurs and the gas turns limewater milky [1]. (b) Add dilute nitric acid, then aqueous barium nitrate [1]; a white precipitate confirms sulfate [1]. The acid removes carbonate ions, which would otherwise give a white precipitate and a false positive [1]. (c) Add dilute nitric acid, then silver nitrate solution [1]; a white precipitate confirms chloride [1]. The acid removes carbonate ions, which would otherwise give a false positive [1].

4. (a) A lit splint [1] — a squeaky pop [1]. (b) A glowing splint [1] — it relights [1]. (c) Bubble through limewater [1] — it turns milky / cloudy [1]. (d) Damp blue litmus paper [1] — it is bleached white [1]. (e) Damp red litmus paper [1] — it turns blue [1].

5. Potassium sulfate, K₂SO₄ [1]. The lilac flame indicates potassium [1]; the white precipitate with acidified barium nitrate indicates sulfate [1].

6. Acidify each solution with dilute nitric acid, then add aqueous silver nitrate to each [1]. Bromide gives a cream precipitate [1]; iodide gives a yellow precipitate [1].

7. Add aqueous ammonia dropwise: copper(II) gives a light blue precipitate and chromium(III) gives a green precipitate [1]. Then add ammonia in excess: the copper(II) precipitate dissolves, giving a dark blue solution [1], while the chromium(III) precipitate is insoluble in excess [1].

8. Add aluminium foil and aqueous sodium hydroxide, then warm [1]. Ammonia gas is released [1], confirmed by turning damp red litmus paper blue [1].

9. (a) Sulfur dioxide, SO₂ [1]. (b) Add a small volume of acidified aqueous potassium manganate(VII) to the solution [1]; it changes colour from purple to colourless [1]. The sulfite ions in the solution react with the manganate(VII) directly: no gas needs to be given off.

10. Dip a clean wire (nichrome or platinum), moistened with hydrochloric acid, into the solid and hold it in a hot (blue) Bunsen flame [1]. Potassium ions give a lilac flame [1].

Examiner insight (June 2025): many candidates did not know the flame colour for potassium. Learn all six: lithium red, sodium yellow, potassium lilac, calcium orange-red, barium light green, copper(II) blue-green. Try the real question next: Cambridge IGCSE Chemistry 0620, June 2025, Paper 32, Question 6.

11. Acidify with dilute nitric acid, then add aqueous silver nitrate [1]; a white precipitate (of silver chloride) forms [1].

Try the real question next: Cambridge O Level Chemistry 5070, June 2025, Paper 22, Question 1.

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Where marks are usually lost

• Forgetting to acidify before testing for sulfate or chloride.
• Giving the test without the positive observation.
• Not using damp litmus paper for gas tests.
• Confusing the Fe²⁺ (green, turning brown near the surface on standing) and Fe³⁺ (red-brown) precipitates.
• Confusing the bromide (cream) and iodide (yellow) precipitates with silver nitrate — the colours are close and worth memorising as a pair.
• Stopping at the dropwise-ammonia stage when a question specifically asks how to distinguish two similar-looking hydroxide precipitates — the distinguishing observation is often in the excess-reagent stage (for example, zinc and aluminium hydroxides are both white until excess ammonia is added).
• Forgetting that barium also gives a flame test result (light green), distinct from calcium’s orange-red — the two are sometimes confused since both are Group 2 metals.
• Confusing the gas test with the ion test: sulfur dioxide gas and sulfite ions in solution both decolourise acidified potassium manganate(VII), but the sulfite test is done on the solution itself. The nitrate test is the one that needs a reaction step first (warm with aqueous sodium hydroxide and aluminium foil, then test the gas for ammonia).

For condensed recall notes on this topic, see the Identification of Ions and Gases revision notes; for the full explanation with the complete cation and anion tables, see the Identification of Ions and Gases study guide.