A student tests an unknown solution to identify the ions present. When sodium hydroxide solution is added dropwise and then in excess, a white precipitate forms that dissolves in excess sodium hydroxide solution. When dilute hydrochloric acid followed by barium chloride solution is added to a separate portion of the unknown solution, a white precipitate forms. Identify the two ions present in the unknown solution and, for each ion, state the observation that allows you to identify it.

Pearson Edexcel International GCSE Chemistry (4CH1) — 2.5 Tests for ions and gases · Identify · 4 marks · View as Markdown

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

A student has an unknown solution that may contain metal cations and anions. The student carries out two separate tests on the solution.

Model answer (4 marks)

The cation is aluminium ions (Al³⁺). The white precipitate that forms when NaOH is added dissolves in excess NaOH, which is characteristic of Al(OH)₃.
The anion is sulfate ions (SO₄²⁻). A white precipitate of BaSO₄ forms when BaCl₂ is added to the acidified solution, which identifies SO₄²⁻.

Examiner tips

  • Use the exact wording: 'Al³⁺' and 'SO₄²⁻'.
  • Explain the key observation for each ion – dissolution in excess NaOH and BaSO₄ formation after acidification.
  • Show the sequence of tests clearly to avoid confusion.

Common mistakes

  • Writing 'Al³⁺' as 'Al' or 'Al³⁺' without the hydroxide dissolution detail.
  • Confusing the precipitate with Ca(OH)₂ or other hydroxides.
  • Failing to mention the acidification step before BaCl₂ for sulfate detection.

Mark scheme (4 marks)

  1. The cation is aluminium ions (Al³⁺)
  2. The white precipitate dissolves in excess sodium hydroxide solution, identifying it as aluminium ions
  3. The anion is sulfate ions (SO₄²⁻)
  4. A white precipitate forms when barium chloride solution is added (after acidification with dilute hydrochloric acid), identifying it as sulfate ions

Key terms in this question

precipitate · sodium hydroxide · barium chloride

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