A forensic scientist carries out qualitative analysis on a white powder dissolved in water to identify the ions present. When sodium hydroxide solution is added to the sample, a white precipitate forms that dissolves in excess sodium hydroxide solution. When dilute nitric acid followed by silver nitrate solution is added to a separate portion of the sample, a cream precipitate forms. Explain what these observations tell the scientist about the identity of the ions present in the white powder.

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

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

Qualitative analysis involves identifying the ions present in an unknown sample using a series of chemical tests.

Model answer (4 marks)

The white precipitate that forms on adding NaOH is Al(OH)₃, which is characteristic of Al³⁺ ions. The fact that it dissolves in excess NaOH confirms that it is aluminium hydroxide rather than a precipitate of e.g. Ca(OH)₂ or Mg(OH)₂, which would not dissolve.

When a separate portion is acidified with HNO₃ and then treated with AgNO₃ a cream‑coloured precipitate of AgBr is produced. This shows that Br⁻ ions are present. The acidification step removes carbonate or other interfering anions that could give a false silver‑salt precipitate.

Thus the white powder contains Al³⁺ and Br⁻ ions.

Examiner tips

  • Mention the specific compounds (Al(OH)₃ and AgBr) and the dissolution test for aluminium.
  • Explain why acidification with HNO₃ is necessary before adding AgNO₃.
  • Use the exact terminology (e.g. ‘white precipitate’, ‘cream precipitate’, ‘dissolves in excess NaOH’).

Common mistakes

  • Confusing the white precipitate with Ca(OH)₂ or Mg(OH)₂; not noting the dissolution test.
  • Failing to state that the cream precipitate is AgBr and that acidification removes interfering ions.
  • Using vague terms like ‘hydroxide precipitate’ without specifying aluminium hydroxide.

Mark scheme (4 marks)

  1. The white precipitate with sodium hydroxide solution indicates the presence of aluminium ions (Al³⁺)
  2. The white precipitate dissolves in excess sodium hydroxide solution, confirming aluminium ions rather than other ions that give white precipitates
  3. The cream precipitate with acidified silver nitrate solution indicates the presence of bromide ions (Br⁻)
  4. Dilute nitric acid is added first to remove carbonate or other interfering ions so that a false positive result is avoided

Key terms in this question

qualitative analysis · precipitate · silver nitrate solution

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