# A forensic scientist carries out qualitative analysis on a white powder dissolved in water to identify the ions present. When sodium hydroxide solution is added to the sample, a white precipitate forms that dissolves in excess sodium hydroxide solution. When dilute nitric acid followed by silver nitrate solution is added to a separate portion of the sample, a cream precipitate forms. Explain what these observations tell the scientist about the identity of the ions present in the white powder.

> Edexcel GCSE Chemistry (1CH0) — 9.1 Qualitative analysis: tests for ions (Chem only) · Explain · 4 marks

> Qualitative analysis involves identifying the ions present in an unknown sample using a series of chemical tests.

## Mark scheme (4 marks)

1. The white precipitate with sodium hydroxide solution indicates the presence of aluminium ions (Al³⁺)
2. The white precipitate dissolves in excess sodium hydroxide solution, confirming aluminium ions rather than other ions that give white precipitates
3. The cream precipitate with acidified silver nitrate solution indicates the presence of bromide ions (Br⁻)
4. Dilute nitric acid is added first to remove carbonate or other interfering ions so that a false positive result is avoided

## Key terms

- [qualitative analysis](https://www.gradenine.co.uk/glossary/qualitative-analysis)
- [precipitate](https://www.gradenine.co.uk/glossary/precipitate)
- [silver nitrate solution](https://www.gradenine.co.uk/glossary/silver-nitrate-solution)

## Related

- [Revision notes for Edexcel GCSE Chemistry (1CH0)](https://www.gradenine.co.uk/learn)
- [How to answer "Explain" questions](https://www.gradenine.co.uk/tools/command-word-cheatsheet)
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Source: [GradeNine](https://www.gradenine.co.uk/q/a-forensic-scientist-carries-out-qualitative-6c80f8bf) · Published by Druglandscape Ltd.