A student carries out qualitative analysis on an unknown solution. When a few drops of sodium hydroxide solution are added, a green precipitate forms. When dilute hydrochloric acid followed by barium chloride solution is added to a fresh sample of the unknown solution, a white precipitate forms. Identify the cation and the anion present in the unknown solution, and describe what these observations show.

Edexcel GCSE Chemistry (1CH0) — 9.1 Qualitative analysis: tests for ions (Chem only) · Describe · 4 marks · View as Markdown

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

A student is given an unknown solution and is asked to identify the ions it contains using qualitative analysis tests.

Model answer (4 marks)

The cation is Fe²⁺.
The green precipitate is iron(II) hydroxide, Fe(OH)₂, formed when OH⁻ reacts with Fe²⁺.
The anion is SO₄²⁻.
The white precipitate with BaCl₂ is barium sulfate, BaSO₄, confirming the presence of sulfate ions.

Examiner tips

  • Use the exact ion names (Fe²⁺, SO₄²⁻) and the chemical formula of the precipitates.
  • Explain the reaction that gives each precipitate – this shows understanding of the test.
  • Keep the answer concise – 4 marks, so one sentence per point is sufficient.

Common mistakes

  • Writing Fe³⁺ instead of Fe²⁺; the green precipitate is Fe(OH)₂, not Fe(OH)₃.
  • Confusing the precipitate with iron(III) hydroxide or iron(II) sulfate.
  • Omitting the confirmation step with BaCl₂ – the white precipitate is key evidence for sulfate.

Mark scheme (4 marks)

  1. The cation is Fe²⁺ (iron(II) ions)
  2. The green precipitate is iron(II) hydroxide, formed when OH⁻ ions react with Fe²⁺ ions
  3. The anion is SO₄²⁻ (sulfate ions)
  4. The white precipitate with barium chloride solution confirms the presence of sulfate ions (barium sulfate is formed)

Key terms in this question

qualitative analysis · precipitate · cation · anion

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