IGCSE Chemistry: Identification of Ions and Gases — Practice Questions

These are original questions written for Marlbridge, in the style and at the standard of the examination. They are not reproduced past-paper questions — 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

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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 a single reagent. [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 colourless gas turns acidified potassium manganate(VII) from purple to colourless. Suggest what the gas is, and explain why a solid giving off this gas would also decolourise the same reagent in solution. [3]

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Answers

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

2. (a) Cu²⁺ — 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 excess sodium hydroxide [1]: the zinc hydroxide dissolves to give a colourless solution, whereas aluminium hydroxide also dissolves — so instead use excess ammonia, in which zinc hydroxide dissolves but aluminium hydroxide does not [1]. (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, while chromium(III) gives a green precipitate, so the two ions can already be distinguished by colour at this stage [1]. Add ammonia in excess to confirm: the copper(II) precipitate dissolves to a deep (royal) blue solution, while the chromium(III) precipitate remains insoluble [1] — the colour and solubility of the excess-ammonia stage confirms the identification [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. The gas is sulfur dioxide [1]. A sulfite solid reacting with acid produces sulfur dioxide gas [1]; this is the same reducing gas that decolourises acidified potassium manganate(VII), so the solid gives the same positive result via the gas it releases [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 almost always in the excess-reagent stage, not the first drops.
• 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.
• Testing the solid itself for a gas, rather than testing the gas that solid releases on reaction — a nitrate or sulfite test always involves a reaction step first.

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.