# Explain how paper chromatography can be used to determine whether a coloured substance is pure or a mixture, and how the Rf value helps to identify a substance.

> Eduqas A-Level Chemistry — 1.7 Equilibria · Explain · 5 marks

> A student carries out paper chromatography on two food dyes, labelled X and Y. Dye X produces a single spot and dye Y produces three spots after the solvent has moved up the paper.

## Mark scheme (5 marks)

1. A pure substance produces a single spot in paper chromatography
2. A mixture produces more than one spot, showing it contains more than one compound
3. Substances separate because they have different solubilities in the mobile phase / they are distributed differently between the mobile phase and the stationary phase
4. The Rf value is calculated as the distance travelled by the substance divided by the distance travelled by the solvent
5. The Rf value can be compared to known values to identify the substance / different compounds have different Rf values in the same solvent

## Key terms

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

## Related

- [Revision notes for Eduqas A-Level Chemistry](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/explain-how-paper-chromatography-can-be-578d5957) · Published by Druglandscape Ltd.