# A student carries out a series of tests to identify the ions present in an unknown solution. The student adds sodium hydroxide solution to the unknown solution and observes a white precipitate. When excess sodium hydroxide solution is added, the white precipitate dissolves. The student then tests the original solution with dilute hydrochloric acid followed by barium chloride solution and observes a white precipitate. Explain what these observations tell the student about the ions present in the unknown solution.

> Edexcel A-Level Chemistry (9CH0) — 4.3 Qualitative analysis of ions · Explain · 5 marks

> A student has an unknown solution that may contain one or more of the following ions: Al³⁺, Ca²⁺, Cu²⁺, SO₄²⁻, Cl⁻. The student performs two separate tests on samples of the unknown solution.

## Mark scheme (5 marks)

1. The white precipitate with sodium hydroxide indicates the presence of a metal cation that forms an insoluble hydroxide
2. The white precipitate dissolving in excess sodium hydroxide identifies the cation as aluminium ions (Al³⁺), because aluminium hydroxide dissolves in excess NaOH whereas calcium hydroxide does not
3. Adding dilute hydrochloric acid before barium chloride solution rules out carbonate or sulfite ions / ensures any interfering ions are removed before the sulfate test
4. The white precipitate with barium chloride solution indicates the presence of sulfate ions (SO₄²⁻)
5. Therefore the unknown solution contains aluminium ions (Al³⁺) and sulfate ions (SO₄²⁻)

## 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 Edexcel A-Level Chemistry (9CH0)](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-student-carries-out-a-series-0c0ab7f1) · Published by Druglandscape Ltd.