# A student uses paper chromatography to investigate the dyes present in three different orange food colourings labelled P, Q and R. The student also runs a sample of a known pure dye called sunset yellow alongside the food colourings. After the experiment, the student measures the distances travelled by each spot and by the solvent front. Explain how the student can use the results of this chromatography experiment to determine whether sunset yellow is present in each of the three food colourings, and explain what it means if a food colouring produces only one spot on the chromatogram.

> OCR A-Level Chemistry B: Salters (H433) — CD Colour by design · Explain · 5 marks

> Paper chromatography can be used to identify the dyes present in food colourings. A reference substance is a known pure compound run alongside unknown samples so that comparisons can be made.

## Mark scheme (5 marks)

1. The student calculates the Rf value for each spot (distance travelled by spot divided by distance travelled by solvent front)
2. If the Rf value of a spot in P, Q or R matches the Rf value of the sunset yellow spot, then sunset yellow is present in that food colouring
3. The solvent carries different dyes different distances because the dyes have different solubilities in the solvent (or different attractions to the paper/stationary phase)
4. If a food colouring produces only one spot, it contains only one dye / it is a pure single dye (it is not a mixture of dyes)
5. If a food colouring produces more than one spot, it is a mixture of dyes

## Key terms

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

## Related

- [Revision notes for OCR A-Level Chemistry B: Salters (H433)](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-7ab60d09) · Published by Druglandscape Ltd.