# A water quality technician tests a sample of water taken from a borehole. The technician carries out a series of tests on the water sample and records the following observations: a white precipitate forms when sodium hydroxide solution is added; the white precipitate dissolves when excess sodium hydroxide solution is added; a white precipitate forms when dilute hydrochloric acid followed by barium chloride solution is added. Identify the cation and the anion present in the borehole water, and explain how each test result supports your identification.

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

> Borehole water can contain dissolved mineral salts. Qualitative analysis tests allow technicians to identify which ions are present in a water sample.

## Mark scheme (5 marks)

1. The cation is aluminium (Al³⁺)
2. A white precipitate with NaOH indicates a metal cation that forms an insoluble metal hydroxide
3. The precipitate dissolving in excess NaOH confirms the cation is aluminium (amphoteric / reacts with excess alkali)
4. The anion is sulfate (SO₄²⁻)
5. A white precipitate with barium chloride solution (after acidification with dilute hydrochloric acid) confirms the presence of sulfate ions, forming barium sulfate

## Key terms

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

## 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-water-quality-technician-tests-a-316f6d68) · Published by Druglandscape Ltd.