# A student carries out paper chromatography on two food colourings, sample A and sample B, using the same solvent. Sample A produces three spots and sample B produces one spot. Explain what these results tell the student about each sample, and describe how the student could use Rf values to help identify the substances present.

> WJEC A-Level Chemistry (Wales) — 1.1 Atoms, molecules and stoichiometry · Explain · 5 marks

> Paper chromatography is used to separate and identify substances in a mixture. The stationary phase is the paper and the mobile phase is the solvent.

## Mark scheme (5 marks)

1. Sample A is a mixture (because it produces more than one spot / three spots)
2. Sample B is a pure substance (because it produces only one spot)
3. The Rf value is calculated as the distance travelled by the substance divided by the distance travelled by the solvent
4. The Rf value of each spot can be compared to known/reference Rf values (of known substances in the same solvent) to identify the substances
5. Different compounds have different Rf values (in a given solvent), so each spot in sample A can be identified separately

## 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 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-954eb4fd) · Published by Druglandscape Ltd.