# A student uses paper chromatography to identify the dyes present in a food colouring. The chromatography is carried out using a pencil line as the baseline. After the experiment, the student calculates the Rf value for each spot. Explain how paper chromatography separates the dyes in the food colouring, and state what the Rf value of a substance is.

> WJEC A-Level Chemistry (Wales) — 3.7 Spectroscopy and chromatography · Explain · 5 marks

> Paper chromatography can be used to separate and identify substances in a mixture. A pencil baseline is drawn near the bottom of the chromatography paper, and a spot of the mixture is placed on it. The paper is then placed in a solvent, which travels up the paper by capillary action.

## Mark scheme (5 marks)

1. The solvent travels up the paper, carrying the dyes with it.
2. Different dyes have different solubilities in the solvent (or different attractions to the paper/solvent).
3. More soluble dyes travel further up the paper, separating the mixture into distinct spots.
4. The Rf value is the distance travelled by the substance divided by the distance travelled by the solvent front.
5. The Rf value is unique to a substance (under the same conditions) and can be used to identify the dye.

## Key terms

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

## 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-uses-paper-chromatography-to-7d1052f5) · Published by Druglandscape Ltd.