# A chemist is investigating whether a sample of red food colouring is a pure substance or a mixture. She carries out paper chromatography on the sample using water as the solvent. After the experiment, she observes three separate spots on the paper. Explain what these results tell the chemist about the food colouring, and explain how paper chromatography produces this separation.

> WJEC GCSE Chemistry (Wales) — 1.1 The nature of substances and chemical reactions · Explain · 4 marks

## Mark scheme (4 marks)

1. Three spots show the food colouring is a mixture (not a pure substance) / contains three different substances/compounds
2. A pure substance would produce only one spot
3. The solvent (mobile phase) moves up the paper carrying the substances in the mixture with it
4. The substances separate because they have different solubilities in the solvent / travel different distances because more soluble substances travel further up the paper

## Key terms

- [paper chromatography](https://www.gradenine.co.uk/glossary/paper-chromatography)
- [pure substance](https://www.gradenine.co.uk/glossary/pure-substance)
- [mixture](https://www.gradenine.co.uk/glossary/mixture)

## Related

- [Revision notes for WJEC GCSE Chemistry (Wales)](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-chemist-is-investigating-whether-a-18de981c) · Published by Druglandscape Ltd.