A forensic scientist tests two white powders, substance A and substance B, using precipitation reactions to identify the ions present. Substance A produces a white precipitate with sodium hydroxide solution that dissolves in excess sodium hydroxide solution. Substance B produces a cream precipitate with dilute nitric acid and silver nitrate solution. Identify the ions present in substance A and substance B, and name the precipitates formed in each test.

AQA GCSE Chemistry (8462) — 4.8.3 Identification of ions by chemical and spectroscopic means (Chem only) · Identify · 4 marks · View as Markdown

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

Precipitation reactions can be used to identify ions in unknown substances. Adding sodium hydroxide solution tests for metal cations, whilst adding silver nitrate solution in the presence of dilute nitric acid tests for halide anions.

Model answer (4 marks)

Substance A:
1. Contains Al³⁺.
2. Precipitate: Al(OH)₃, white, dissolves in excess NaOH.

Substance B:
1. Contains Br⁻.
2. Precipitate: AgBr, cream coloured, formed with dilute HNO₃ and AgNO₃.

Examiner tips

  • Use the exact ion symbols (Al³⁺, Br⁻) and precipitate names (Al(OH)₃, AgBr).
  • Show the colour of the precipitate for B – cream – to gain the full mark.
  • Mention that the Al(OH)₃ dissolves in excess NaOH to confirm the ion.

Common mistakes

  • Writing AlOH instead of Al(OH)₃ or omitting the charge on Al³⁺.
  • Calling the B precipitate ‘silver chloride’ instead of AgBr.
  • Failing to state that the Al(OH)₃ dissolves in excess NaOH.

Mark scheme (4 marks)

  1. Substance A contains aluminium ions (Al³⁺)
  2. The precipitate formed with substance A is aluminium hydroxide (which dissolves in excess NaOH)
  3. Substance B contains bromide ions (Br⁻)
  4. The precipitate formed with substance B is silver bromide (AgBr)

Key terms in this question

precipitation · precipitate · ions

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