# 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

> 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.

## 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

- [cation](https://www.gradenine.co.uk/glossary/cation)
- [anion](https://www.gradenine.co.uk/glossary/anion)
- [flame test](https://www.gradenine.co.uk/glossary/flame-test)
- [precipitate](https://www.gradenine.co.uk/glossary/precipitate)

## Related

- [Revision notes for Edexcel A-Level Chemistry (9CH0)](https://www.gradenine.co.uk/learn)
- [How to answer "Explain" questions](https://www.gradenine.co.uk/tools/command-word-cheatsheet)
- [Practice this with AI marking (free)](https://www.gradenine.co.uk/start)

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Source: [GradeNine](https://www.gradenine.co.uk/q/a-forensic-scientist-analyses-a-white-07e64967) · Published by Druglandscape Ltd.