# A student tests an unknown solution to identify the ions present. When sodium hydroxide solution is added dropwise and then in excess, a white precipitate forms that dissolves in excess sodium hydroxide solution. When dilute hydrochloric acid followed by barium chloride solution is added to a separate portion of the unknown solution, a white precipitate forms. Identify the two ions present in the unknown solution and, for each ion, state the observation that allows you to identify it.

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

> A student has an unknown solution that may contain metal cations and anions. The student carries out two separate tests on the solution.

## Mark scheme (4 marks)

1. The cation is aluminium ions (Al³⁺)
2. The white precipitate dissolves in excess sodium hydroxide solution, identifying it as aluminium ions
3. The anion is sulfate ions (SO₄²⁻)
4. A white precipitate forms when barium chloride solution is added (after acidification with dilute hydrochloric acid), identifying it as sulfate ions

## Key terms

- [precipitate](https://www.gradenine.co.uk/glossary/precipitate)
- [sodium hydroxide](https://www.gradenine.co.uk/glossary/sodium-hydroxide)
- [barium chloride](https://www.gradenine.co.uk/glossary/barium-chloride)

## Related

- [Revision notes for Pearson Edexcel International GCSE Chemistry (4CH1)](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)

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Source: [GradeNine](https://www.gradenine.co.uk/q/a-student-tests-an-unknown-solution-ecefbb63) · Published by Druglandscape Ltd.