A student heats a sample of an unknown metal compound on a nichrome wire loop in a flame and observes a lilac colour. The student then adds dilute nitric acid followed by silver nitrate solution to a solution of the same compound and observes a pale yellow precipitate. Explain what these two observations tell the student about the identity of the compound.

OCR GCSE Chemistry A: Gateway Science (J248) — C4.2 Identifying the products of chemical reactions · Explain · 4 marks · View as Markdown

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

Flame tests and precipitation reactions are qualitative tests used to identify the ions present in an unknown compound.

Model answer (4 marks)

The lilac flame colour shows that potassium ions are present, because only K⁺ gives a lilac flame.
The flame test is a qualitative test used to identify metal cations.
The pale yellow precipitate that forms when silver nitrate is added is AgBr, so the solution contains bromide ions.
Therefore the unknown compound must be potassium bromide, KBr.

Examiner tips

  • Use the command word ‘explain’ to describe the reasoning, not just state facts.
  • Show the link between flame colour and K⁺, and between Ag⁺ + Br⁻ → AgBr.
  • Mention that the test is qualitative and that the precipitate confirms Br⁻.
  • Conclude with the full formula KBr.

Mark scheme (4 marks)

  1. The lilac flame colour identifies the presence of potassium ions (cations)
  2. The flame test is a qualitative test used to identify metal ions (cations)
  3. The pale yellow precipitate with silver nitrate solution identifies bromide ions (halide ions / Br⁻)
  4. Therefore the compound is potassium bromide

Key terms in this question

precipitate

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