# A student uses paper chromatography to investigate the dyes present in two food colourings, labelled P and Q. Food colouring P produces three spots on the paper. Food colouring Q produces one spot. The student claims that food colouring Q is a pure substance. Explain how paper chromatography separates the dyes in a mixture, and evaluate whether the student's claim about food colouring Q is correct.

> Eduqas A-Level Chemistry — 1.6 Kinetics · Explain and Evaluate · 5 marks

## Mark scheme (5 marks)

1. In paper chromatography, the solvent acts as the mobile phase and carries the dyes up the paper (stationary phase).
2. Dyes that are more soluble in the solvent travel further up the paper, so dyes with different solubilities separate into different spots.
3. Food colouring P is a mixture because it produces more than one spot.
4. A pure compound produces a single spot in all solvents, so the student's claim could be correct if Q gives one spot in all solvents tested.
5. However, one spot in a single solvent does not conclusively prove Q is pure; the test should be repeated in different solvents to confirm Q is a pure substance.

## Key terms

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

## Related

- [Revision notes for Eduqas A-Level Chemistry](https://www.gradenine.co.uk/learn)
- [How to answer "Explain and Evaluate" 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-51a95807) · Published by Druglandscape Ltd.