# A student carries out paper chromatography on a sample of food colouring to find out whether it is a pure substance or a mixture. The food colouring produces three separate spots on the paper. Explain what this result tells the student about the food colouring, and describe how the student could use Rf values to identify each of the substances present.

> WJEC A-Level Chemistry (Wales) — 1.5 Energetics · Explain · 5 marks

> Paper chromatography separates substances in a mixture based on how soluble each substance is in the mobile phase. The more soluble a substance is, the further up the paper it travels.

## Mark scheme (5 marks)

1. Three separate spots show the food colouring is a mixture (not a pure substance)
2. Each spot represents a different compound/substance in the mixture
3. Rf value is calculated as the distance travelled by the dissolved substance divided by the distance travelled by the solvent
4. An Rf value is calculated for each of the three spots
5. The Rf values are compared to known/reference Rf values (in the same solvent) to identify each substance

## 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)
- [Rf value](https://www.gradenine.co.uk/glossary/rf-value)

## 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-carries-out-paper-chromatography-4195cb1e) · Published by Druglandscape Ltd.