A chemist is analysing a white powder suspected to contain either calcium ions, copper(II) ions or iron(III) ions, as well as either chloride, sulfate or carbonate anions. The chemist dissolves the powder in dilute hydrochloric acid and carries out a series of tests to identify both the cation and the anion present. Describe the observations the chemist would make when carrying out a flame test on the original powder, and state what these observations indicate about which cation is present. Then describe the test and expected observation that would confirm the presence of sulfate ions in the solution.

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

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

A white powder is dissolved in dilute hydrochloric acid to form a colourless solution. A flame test is then performed on the original white powder. To test for the anion, a few drops of dilute hydrochloric acid followed by a few drops of barium chloride solution are added to a portion of the solution.

Model answer (4 marks)

Flame test: the powder gives a bright brick‑red/orange‑red flame.

This colour is characteristic of Ca²⁺, so the cation present is calcium.

Sulfate test: add a few drops of dilute HCl to acidify the solution, then add BaCl₂.
A white precipitate of BaSO₄ forms, confirming the presence of sulfate ions.

Examiner tips

  • Mention the flame colour and link it to Ca²⁺; use the exact colour description. State the sequence of additions (HCl then BaCl₂) and the white precipitate. Explain that the precipitate is BaSO₄, which proves sulfate is present.

Common mistakes

  • Writing the flame colour as orange or red only, missing the brick‑red detail. Forgetting to acidify the solution before adding BaCl₂. Failing to identify the precipitate as BaSO₄ rather than just a white solid.

Mark scheme (4 marks)

  1. The flame test produces a brick-red / orange-red flame, indicating the presence of calcium ions
  2. Add dilute hydrochloric acid (to acidify the solution) followed by barium chloride solution
  3. A white precipitate forms
  4. The white precipitate is barium sulfate, confirming sulfate ions are present

Key terms in this question

flame test · sulfate ions · calcium ions

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