# A student carries out tests on an unknown solution to identify the anions present. First, dilute nitric acid is added, followed by silver nitrate solution. A cream-coloured precipitate forms. In a separate test, dilute hydrochloric acid is added to a fresh sample of the solution, followed by barium chloride solution. A white precipitate forms that does not dissolve in excess dilute hydrochloric acid. State the identity of the two anions present in the unknown solution and explain how each test result leads to that conclusion.

> Pearson Edexcel International GCSE Chemistry (4CH1) — 2.5 Tests for ions and gases · State and Explain · 4 marks

## Mark scheme (4 marks)

1. The cream precipitate with acidified silver nitrate solution indicates the presence of bromide ions (Br⁻)
2. Silver nitrate is added after dilute nitric acid to prevent interference from other anions / ensure the precipitate is due to the halide only
3. The white precipitate that does not dissolve in excess dilute hydrochloric acid with barium chloride solution indicates the presence of sulfate ions (SO₄²⁻)
4. Dilute hydrochloric acid is added first to ensure the precipitate is barium sulfate and not barium carbonate or barium sulfite (which would dissolve in the acid)

## Key terms

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

## Related

- [Revision notes for Pearson Edexcel International GCSE Chemistry (4CH1)](https://www.gradenine.co.uk/learn)
- [How to answer "State and 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-student-carries-out-tests-on-599c884a) · Published by Druglandscape Ltd.