A student carries out a flame test on an unknown solution and observes a lilac flame. The student then adds dilute hydrochloric acid followed by barium chloride solution to a separate sample of the unknown solution, producing a white precipitate. Identify the metal ion and the negative ion present in the unknown solution. Explain how the results of the two tests support your answers.

AQA GCSE Chemistry (8462) — 4.8.3 Identification of ions by chemical and spectroscopic means (Chem only) · Explain · 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 is potassium.

A lilac flame indicates the presence of K⁺.

The negative ion is sulfate.

Adding HCl followed by BaCl₂ gives a white BaSO₄ precipitate, confirming SO₄²⁻.

Examiner tips

  • Use the exact flame colour terminology (lilac/purple) to show knowledge of K⁺. Show the sequence of tests and link each result to the ion. Mention the white precipitate as BaSO₄ to prove sulfate.

Common mistakes

  • Confusing lilac with orange or red flame colours. Failing to specify that the precipitate is BaSO₄. Not linking the HCl addition to the precipitation step.

Mark scheme (4 marks)

  1. The metal ion is potassium (K⁺)
  2. A lilac / purple flame colour indicates the presence of potassium ions
  3. The negative ion is sulfate (SO₄²⁻)
  4. A white precipitate with barium chloride solution (after adding hydrochloric acid) indicates the presence of sulfate ions

Key terms in this question

flame test · metal ion · white precipitate

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