A student has three unlabelled solutions: copper(II) sulfate, iron(III) chloride, and zinc sulfate. Describe how the student could use aqueous ammonia to distinguish between all three solutions. Include the observations that would confirm the identity of each solution.

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)

Add excess aqueous ammonia to each solution.
1. Copper(II) sulfate: a pale blue precipitate of Cu(OH)₂ forms, then dissolves in excess ammonia to give a deep blue solution of [Cu(NH₃)₄]²⁺.
2. Iron(III) chloride: a red‑brown/orange‑brown precipitate of Fe(OH)₃ forms, which does not dissolve in excess ammonia.
3. Zinc sulfate: a white precipitate of Zn(OH)₂ forms, which dissolves in excess ammonia to give a colourless solution.

Examiner tips

  • Describe the addition of excess ammonia to each solution; mention the colour change for each ion.
  • Show the sequence: precipitate forms then dissolves (or not) to confirm identity.
  • Use the exact colour terms (pale blue, deep blue, red‑brown, white, colourless).

Common mistakes

  • Failing to note that the copper precipitate dissolves in excess ammonia.
  • Confusing the iron precipitate as dissolving when it actually remains solid.
  • Using vague colour descriptions such as ‘blue’ instead of ‘pale blue’ or ‘deep blue’.

Mark scheme (5 marks)

  1. Add excess aqueous ammonia to each solution (award for describing addition of aqueous ammonia to each solution)
  2. Copper(II) sulfate: pale blue / light blue precipitate forms initially
  3. Copper(II) sulfate: precipitate dissolves in excess ammonia to give a deep blue / dark blue solution
  4. Iron(III) chloride: red-brown / orange-brown precipitate that does not dissolve in excess ammonia
  5. Zinc sulfate: white precipitate that dissolves in excess ammonia to give a colourless solution

Key terms in this question

aqueous ammonia · copper(II) sulfate · iron(III) chloride · zinc sulfate

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