# A chemist uses paper chromatography to separate and identify the dyes in a food colouring mixture. Explain how paper chromatography works and how the chemist can use the results to identify the dyes present.

> Eduqas A-Level Chemistry — 3.7 Spectroscopy and chromatography · Explain · 5 marks

> Paper chromatography is a technique used to separate mixtures of substances. A spot of the food colouring mixture is placed on the baseline of a piece of chromatography paper and placed in a solvent. A reference sample of known dyes is run alongside the mixture. After the solvent has moved up the paper, the chromatogram is analysed.

## Mark scheme (5 marks)

1. The solvent moves up the paper (by capillary action / is absorbed into the paper)
2. Different dyes/substances travel different distances up the paper because they have different solubilities in the solvent / different attractions to the paper
3. Each dye produces a spot at a particular distance from the baseline, giving a separated chromatogram
4. The Rf value is calculated as the distance travelled by the spot divided by the distance travelled by the solvent front
5. The Rf value (or spot position) of each dye in the mixture is compared to the Rf value (or spot position) of the known reference dyes to identify which dyes are present

## Key terms

- [chromatography](https://www.gradenine.co.uk/glossary/chromatography)

## 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)

---
Source: [GradeNine](https://www.gradenine.co.uk/q/a-chemist-uses-paper-chromatography-to-b6cc85d7) · Published by Druglandscape Ltd.