# A student carries out paper chromatography to investigate the dyes present in three food colourings labelled P, Q and R. Food colouring P produces one spot, food colouring Q produces three spots and food colouring R produces two spots. The student wants to identify the individual dyes by comparing their Rf values with reference data. Explain what the results tell the student about the nature of each food colouring, and explain how Rf values can be used to identify the dyes present.

> Eduqas A-Level Chemistry — 1.4 States of matter · Explain · 5 marks

> Food colouring P produces one spot, food colouring Q produces three spots and food colouring R produces two spots during paper chromatography using the same solvent.

## Mark scheme (5 marks)

1. Food colouring P is a pure substance (contains only one compound / single dye) because it produces only one spot
2. Food colourings Q and R are mixtures because they produce more than one spot, showing they contain more than one compound / dye
3. In paper chromatography, substances separate because they have different solubilities in the mobile phase (solvent), so they travel different distances up the paper
4. The Rf value is calculated as the distance travelled by the dissolved substance divided by the distance travelled by the solvent
5. Each dye has a characteristic / unique Rf value in a given solvent, so the Rf values of the spots can be compared to reference / known Rf values to identify the dyes

## 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/a-student-carries-out-paper-chromatography-ad546cff) · Published by Druglandscape Ltd.