A student adds a few drops of dilute sodium hydroxide solution to separate aqueous solutions of iron(II) sulfate and iron(III) sulfate. Describe what the student would observe in each test, and explain what these observations show about the ions present.

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).

Iron(II) ions and iron(III) ions both react with hydroxide ions to form precipitates. These reactions can be used to identify transition metal ions in aqueous solution.

Model answer (5 marks)

Iron(II) sulfate:
1. A green precipitate forms.
2. The precipitate is iron(II) hydroxide, Fe(OH)₂.

Iron(III) sulfate:
3. A red‑brown precipitate forms.
4. The precipitate is iron(III) hydroxide, Fe(OH)₃.

5. Because the two hydroxides have distinct colours, adding NaOH can distinguish Fe²⁺ from Fe³⁺ in solution.

Examiner tips

  • Use the colour of the precipitate as the key observation; link each colour to the correct hydroxide. Keep the explanation short: colour difference shows different oxidation states. Include the chemical formulae to demonstrate understanding.

Common mistakes

  • Confusing the colours (green vs red‑brown).
  • Failing to state the formulae of the hydroxides.

Mark scheme (5 marks)

  1. Iron(II) sulfate produces a green precipitate
  2. The green precipitate is iron(II) hydroxide / Fe(OH)₂
  3. Iron(III) sulfate produces a red-brown precipitate
  4. The red-brown precipitate is iron(III) hydroxide / Fe(OH)₃
  5. The two precipitates have different colours, so sodium hydroxide solution can be used to distinguish between iron(II) ions and iron(III) ions

Key terms in this question

sodium hydroxide solution · aqueous solution

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