# A student is investigating whether a sample of a food dye is a pure substance or a mixture. She dissolves a small amount of the dye in water and carries out paper chromatography. After the experiment, she observes three separate spots on the paper. Explain what the results of the chromatography tell the student about the food dye, and describe how paper chromatography works to produce this result.

> WJEC A-Level Chemistry (Wales) — 1.3 Chemical bonding · Explain · 5 marks

> A student carries out paper chromatography on a food dye sample. After the solvent has travelled up the paper, three separate coloured spots are visible.

## Mark scheme (5 marks)

1. Three spots show the food dye is a mixture (not a pure substance)
2. The food dye contains three different substances / compounds
3. Paper chromatography involves a stationary phase (the paper) and a mobile phase (the solvent)
4. The solvent (mobile phase) carries the substances up the paper
5. The substances separate because they have different solubilities in the solvent / travel different distances because they are more or less soluble

## Key terms

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

## 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-is-investigating-whether-a-cd472103) · Published by Druglandscape Ltd.