# A student uses paper chromatography to investigate whether a food dye is a pure substance or a mixture. The student dissolves the food dye in a solvent and allows the solvent to travel up the paper. Explain how the results of the paper chromatography experiment would allow the student to determine whether the food dye is a pure substance or a mixture.

> WJEC A-Level Chemistry (Wales) — 1.8 Redox · Explain · 5 marks

> A food manufacturer claims that one of its red food dyes is a single pure compound. A student decides to test this claim using paper chromatography.

## Mark scheme (5 marks)

1. The mobile phase (solvent) carries the substances up the stationary phase (paper)
2. Substances in the mixture separate because they have different solubilities in the solvent / travel at different speeds
3. If the food dye is a pure substance, it will produce only one spot
4. If the food dye is a mixture, it will produce more than one spot
5. The Rf value of any spot can be compared to known Rf values to identify the compound(s) present

## 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 A-Level 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-student-uses-paper-chromatography-to-f9755f98) · Published by Druglandscape Ltd.