# A forensic scientist is analysing an unknown white powder found at a crime scene. The scientist dissolves the powder in water and carries out a series of chemical tests to identify the ions present. The scientist adds dilute hydrochloric acid to one portion of the solution and observes a fizzing. When a few drops of silver nitrate solution are then added to a second portion of the original solution, a cream-coloured precipitate forms. Explain what these two observations tell the scientist about the ions present in the unknown powder.

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

> A forensic scientist dissolves an unknown white powder in water and performs chemical tests. Adding dilute hydrochloric acid to the solution produces fizzing. Adding silver nitrate solution to a separate portion of the original solution produces a cream-coloured precipitate.

## Mark scheme (4 marks)

1. The fizzing with dilute hydrochloric acid indicates the presence of carbonate ions (CO₃²⁻)
2. The gas produced in the fizzing is carbon dioxide, which can be confirmed by testing with limewater turning milky/cloudy
3. The cream-coloured precipitate with silver nitrate solution indicates the presence of bromide ions (Br⁻)
4. The cream precipitate is silver bromide, formed because silver ions react with bromide ions

## Key terms

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

## Related

- [Revision notes for OCR GCSE Chemistry A: Gateway Science (J248)](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-is-analysing-an-c11feb2e) · Published by Druglandscape Ltd.