A student carries out qualitative analysis on two unknown solutions, Solution A and Solution B. Solution A gives a brick-red flame colour during a flame test. When sodium hydroxide solution is added to Solution B, a blue precipitate forms. Explain what these observations tell the student about the metal ions present in each solution.

Edexcel GCSE Chemistry (1CH0) — 9.1 Qualitative analysis: tests for ions (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)

Solution A contains Ca²⁺ – the brick‑red flame colour is characteristic of calcium ions, so the flame test identifies the cation as Ca²⁺.
Solution B contains Cu²⁺ – adding NaOH gives a blue precipitate of Cu(OH)₂, confirming the presence of copper ions.

Examiner tips

  • Use the exact flame‑colour and precipitate colour terminology; link each observation to the ion identified.
  • Show the reaction for the precipitate (Cu²⁺ + 2 OH⁻ → Cu(OH)₂).

Common mistakes

  • Confusing the flame colour of sodium (yellow) with calcium; writing ‘sodium’ instead of ‘calcium’.
  • Assuming the blue precipitate is due to iron or other metals; not recognising it as Cu(OH)₂.

Mark scheme (4 marks)

  1. Solution A contains calcium ions (Ca²⁺)
  2. The brick-red flame colour indicates the presence of calcium ions / the flame test identifies the metal cation by the colour produced
  3. Solution B contains copper ions (Cu²⁺)
  4. The blue precipitate forms because copper ions react with hydroxide ions / sodium hydroxide solution is used to identify metal cations by the colour of the precipitate formed

Key terms in this question

flame test · qualitative analysis · precipitate · metal ions

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