A water quality technician tests a sample of water taken from a borehole. The technician carries out a series of tests on the water sample and records the following observations: a white precipitate forms when sodium hydroxide solution is added; the white precipitate dissolves when excess sodium hydroxide solution is added; a white precipitate forms when dilute hydrochloric acid followed by barium chloride solution is added. Identify the cation and the anion present in the borehole water, and explain how each test result supports your identification.

Edexcel A-Level Chemistry (9CH0) — 4.3 Qualitative analysis of ions · Explain · 5 marks · View as Markdown

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

Borehole water can contain dissolved mineral salts. Qualitative analysis tests allow technicians to identify which ions are present in a water sample.

Model answer (5 marks)

The cation present is aluminium (Al³⁺). A white precipitate forms when NaOH is added because aluminium hydroxide, Al(OH)₃, is insoluble. The precipitate dissolves in excess NaOH, showing the amphoteric nature of Al(OH)₃, which reacts with excess alkali to give the aluminate ion, [Al(OH)₄]⁻. The anion present is sulfate (SO₄²⁻). After acidifying the sample with dilute HCl, adding BaCl₂ gives a white precipitate of barium sulfate, BaSO₄, confirming the presence of sulfate ions.

Examiner tips

  • Use the exact terms: aluminium hydroxide, amphoteric, aluminate ion, barium sulfate.
  • Show the sequence of tests and link each observation to the ion identified.
  • Mention the role of acidification before BaCl₂ addition for sulfate detection.
  • Keep the answer concise and directly tied to the marks listed.

Common mistakes

  • Confusing the precipitate with aluminium hydroxide as a generic metal hydroxide without noting its amphoteric behaviour.
  • Forgetting to state that the precipitate dissolves in excess NaOH, which is key evidence for Al³⁺.
  • Mistaking the white precipitate with BaCl₂ as a chloride test rather than a sulfate test after acidification.

Mark scheme (5 marks)

  1. The cation is aluminium (Al³⁺)
  2. A white precipitate with NaOH indicates a metal cation that forms an insoluble metal hydroxide
  3. The precipitate dissolving in excess NaOH confirms the cation is aluminium (amphoteric / reacts with excess alkali)
  4. The anion is sulfate (SO₄²⁻)
  5. A white precipitate with barium chloride solution (after acidification with dilute hydrochloric acid) confirms the presence of sulfate ions, forming barium sulfate

Key terms in this question

cation · precipitate · barium chloride · sodium hydroxide

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