A student investigates the reactions of aqueous ammonium ions and aqueous carbonate ions with various reagents. Describe the observations the student would make when: (i) aqueous sodium hydroxide is added to an ammonium salt solution and the mixture is warmed, and (ii) dilute hydrochloric acid is added to an aqueous solution containing carbonate ions. Include the name of any gas produced in each case.

AQA A-Level Chemistry (7405) — 3.2.6 Reactions of ions in aqueous solution (A-Level only) · Describe · 5 marks · View as Markdown

Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).

Model answer (5 marks)

(i) Adding aqueous NaOH to an ammonium salt solution gives a pungent, sharp smell on warming. The gas released is ammonia (NH₃). Damp red litmus paper turns blue (or damp universal indicator paper turns blue/purple).\n(ii) Adding dilute HCl to a solution containing carbonate ions produces effervescence – bubbles are seen. The gas released is carbon dioxide (CO₂), which can be confirmed by turning limewater milky.

Examiner tips

  • Use the word "pungent" or "sharp" for the smell of NH₃; mention the colour change of red litmus to blue or universal indicator to blue/purple. Include the gas name in each part. For part (ii) state effervescence and CO₂, and optionally the limewater test.
  • Remember to separate the two parts clearly and use the exact terminology expected by the mark scheme.

Mark scheme (5 marks)

  1. (i) A pungent / sharp smell is produced on warming
  2. (i) The gas produced is ammonia
  3. (i) Damp red litmus paper turns blue (accept: damp universal indicator paper turns blue/purple)
  4. (ii) Effervescence / bubbles are observed
  5. (ii) The gas produced is carbon dioxide (accept: CO₂; accept: turns limewater milky as the test)

Key terms in this question

ammonium ions · carbonate ions

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