A student carries out tests on an unknown solution to identify the anions present. First, dilute nitric acid is added, followed by silver nitrate solution. A cream-coloured precipitate forms. In a separate test, dilute hydrochloric acid is added to a fresh sample of the solution, followed by barium chloride solution. A white precipitate forms that does not dissolve in excess dilute hydrochloric acid. State the identity of the two anions present in the unknown solution and explain how each test result leads to that conclusion.

Pearson Edexcel International GCSE Chemistry (4CH1) — 2.5 Tests for ions and gases · State and Explain · 4 marks · View as Markdown

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

Model answer (4 marks)

1. Bromide ions – the cream‑coloured precipitate that appears when silver nitrate is added after acidifying the solution with dilute nitric acid is AgBr, confirming Br⁻.
2. Sulfate ions – the white precipitate that remains insoluble in excess dilute hydrochloric acid after adding barium chloride is BaSO₄, confirming SO₄²⁻.

Examiner tips

  • Use the exact names AgBr and BaSO₄; explain the role of the acid in each step.
  • Show the sequence of reagents to demonstrate why the precipitate is specific to each anion.

Common mistakes

  • Confusing the cream precipitate with AgCl or AgI; not mentioning the acid’s role.
  • Mistaking the white precipitate for barium carbonate or sulfite and claiming it dissolves in acid.

Mark scheme (4 marks)

  1. The cream precipitate with acidified silver nitrate solution indicates the presence of bromide ions (Br⁻)
  2. Silver nitrate is added after dilute nitric acid to prevent interference from other anions / ensure the precipitate is due to the halide only
  3. The white precipitate that does not dissolve in excess dilute hydrochloric acid with barium chloride solution indicates the presence of sulfate ions (SO₄²⁻)
  4. Dilute hydrochloric acid is added first to ensure the precipitate is barium sulfate and not barium carbonate or barium sulfite (which would dissolve in the acid)

Key terms in this question

silver nitrate solution · barium chloride solution · precipitate

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