# A forensic scientist tests two white powders, substance A and substance B, using precipitation reactions to identify the ions present. Substance A produces a white precipitate with sodium hydroxide solution that dissolves in excess sodium hydroxide solution. Substance B produces a cream precipitate with dilute nitric acid and silver nitrate solution. Identify the ions present in substance A and substance B, and name the precipitates formed in each test.

> AQA GCSE Chemistry (8462) — 4.8.3 Identification of ions by chemical and spectroscopic means (Chem only) · Identify · 4 marks

> Precipitation reactions can be used to identify ions in unknown substances. Adding sodium hydroxide solution tests for metal cations, whilst adding silver nitrate solution in the presence of dilute nitric acid tests for halide anions.

## Mark scheme (4 marks)

1. Substance A contains aluminium ions (Al³⁺)
2. The precipitate formed with substance A is aluminium hydroxide (which dissolves in excess NaOH)
3. Substance B contains bromide ions (Br⁻)
4. The precipitate formed with substance B is silver bromide (AgBr)

## Key terms

- [precipitation](https://www.gradenine.co.uk/glossary/precipitation)
- [precipitate](https://www.gradenine.co.uk/glossary/precipitate)
- [ions](https://www.gradenine.co.uk/glossary/ions)

## Related

- [Revision notes for AQA GCSE Chemistry (8462)](https://www.gradenine.co.uk/learn)
- [How to answer "Identify" questions](https://www.gradenine.co.uk/tools/command-word-cheatsheet)
- [Practice this with AI marking (free)](https://www.gradenine.co.uk/start)

---
Source: [GradeNine](https://www.gradenine.co.uk/q/a-forensic-scientist-tests-two-white-347929ec) · Published by Druglandscape Ltd.