A forensic scientist analyses a white powder found at a crime scene. The scientist dissolves the powder in distilled water and carries out a series of chemical tests. First, dilute hydrochloric acid is added, producing a white precipitate. The white precipitate dissolves when excess hydrochloric acid is added. Next, a flame test is carried out on a fresh sample of the solution, producing a lilac flame. Identify the cation and the anion present in the white powder. Explain how the results of each test support 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).

A forensic scientist analyses a white powder found at a crime scene. The scientist dissolves the powder in distilled water and carries out a series of chemical tests. First, dilute hydrochloric acid is added, producing a white precipitate. The white precipitate dissolves when excess hydrochloric acid is added. Next, a flame test is carried out on a fresh sample of the solution, producing a lilac flame.

Model answer (5 marks)

1. The cation is potassium (K⁺). 2. The lilac flame test confirms the presence of K⁺, as potassium ions give a lilac/purple flame. 3. The anion is carbonate (CO₃²⁻). 4. Adding dilute HCl to the solution produces a white precipitate of potassium carbonate (K₂CO₃). 5. The precipitate dissolves when excess HCl is added, which is characteristic of a carbonate; a sulfate would give a white precipitate that remains insoluble in excess acid, so the dissolving precipitate rules out sulfate and confirms carbonate.

Examiner tips

  • Use the command word ‘Explain’ – give a clear statement of the ion, then describe each test result and link it to the ion’s behaviour.
  • Show the logical sequence: flame test → K⁺; HCl test → precipitate → dissolution → carbonate.
  • Mention the key distinguishing feature of carbonate (soluble in excess acid).

Common mistakes

  • Confusing the flame colour – writing ‘red’ instead of lilac for potassium.
  • Assuming the precipitate is a sulfate because it is white, without noting that it dissolves in excess acid.

Mark scheme (5 marks)

  1. The cation is potassium (K⁺)
  2. The lilac flame indicates the presence of potassium ions
  3. The anion is carbonate (CO₃²⁻) / the powder contains a carbonate
  4. A white precipitate that dissolves in excess hydrochloric acid is characteristic of a carbonate
  5. A sulfate would also give a white precipitate with hydrochloric acid but would NOT dissolve in excess, so the soluble precipitate rules out sulfate and confirms carbonate

Key terms in this question

cation · anion · flame test · precipitate

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