# A student has an unknown solution that may contain one or more of the following ions: carbonate ions, sulfate ions, or halide ions. The student adds dilute hydrochloric acid to a sample of the solution and observes effervescence. A second sample is then tested with acidified silver nitrate solution, producing a cream-coloured precipitate. Explain what the student can conclude from these two test results.

> Eduqas GCSE Chemistry — C8 Chemical analysis · Explain · 4 marks

## Mark scheme (4 marks)

1. The effervescence with dilute hydrochloric acid shows that carbonate ions are present
2. The gas produced is carbon dioxide
3. The cream-coloured precipitate with acidified silver nitrate solution indicates bromide ions are present
4. The acid must be added first to remove carbonate ions before testing for halide ions, to avoid a false positive

## Key terms

- [carbonate ions](https://www.gradenine.co.uk/glossary/carbonate-ions)
- [halide ions](https://www.gradenine.co.uk/glossary/halide-ions)
- [silver nitrate solution](https://www.gradenine.co.uk/glossary/silver-nitrate-solution)
- [precipitate](https://www.gradenine.co.uk/glossary/precipitate)
- [effervescence](https://www.gradenine.co.uk/glossary/effervescence)

## Related

- [Revision notes for Eduqas GCSE Chemistry](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-has-an-unknown-solution-0723f0e2) · Published by Druglandscape Ltd.