A geologist finds a mineral sample and dissolves part of it in dilute hydrochloric acid to produce a solution. The geologist carries out qualitative analysis on the solution to identify the ions present. When dilute sodium hydroxide solution is added in excess, a white precipitate forms that dissolves to give a colourless solution. When dilute hydrochloric acid is added to a separate portion of the solution followed by barium chloride solution, a white precipitate forms. Identify the metal ion and the negative ion present in the geologist's solution and describe what each positive result tells the geologist.

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).

Model answer (4 marks)

The metal ion present is aluminium (Al³⁺). The white precipitate that dissolves in excess NaOH shows that the metal ion forms an amphoteric hydroxide (Al(OH)₃) which redissolves, confirming Al³⁺.
The negative ion present is sulfate (SO₄²⁻). The white precipitate that forms with BaCl₂ after acidification confirms the presence of SO₄²⁻.

Examiner tips

  • Use the exact terms ‘aluminium hydroxide’ and ‘sulfate’ – examiners look for correct ion names.
  • Explain that the precipitate dissolving in excess NaOH indicates an amphoteric hydroxide, not a simple hydroxide.
  • Mention that the BaCl₂ test is performed after acidification to avoid interference from carbonate or hydroxide.
  • Show the sequence of tests: NaOH → dissolution, HCl → acidification, BaCl₂ → precipitate.

Common mistakes

  • Writing ‘Al³⁺’ as ‘Al³⁺ ion’ instead of ‘aluminium’
  • Confusing the dissolution test with a simple hydroxide test and not recognising amphoterism
  • Failing to state that the BaCl₂ test is after acidification

Mark scheme (4 marks)

  1. The metal ion present is aluminium (Al³⁺)
  2. The white precipitate dissolving in excess sodium hydroxide solution shows the metal ion is from an amphoteric/aluminium hydroxide precipitate that redissolves, confirming the ion is Al³⁺ (or Zn²⁺)
  3. The negative ion present is sulfate (SO₄²⁻)
  4. The white precipitate with barium chloride solution (after acidifying with dilute hydrochloric acid) confirms the presence of sulfate ions

Key terms in this question

qualitative analysis · precipitate · sodium hydroxide · barium chloride

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