# A student carries out paper chromatography to investigate whether a food dye is a pure substance or a mixture. The food dye produces three separate spots on the paper. Explain what this result tells the student about the food dye, and describe how the student could use Rf values to identify each substance present.

> Eduqas A-Level Chemistry — 1.1 Atoms, molecules and stoichiometry · Explain · 5 marks

## Mark scheme (5 marks)

1. Three spots indicate the food dye is a mixture (not a pure substance)
2. A pure compound would produce only a single spot (in all solvents)
3. The Rf value is calculated as the distance travelled by the substance divided by the distance travelled by the solvent
4. Each spot gives a different Rf value (because each substance travels a different distance)
5. The Rf values can be compared to known/reference Rf values to identify each substance in the mixture

## Key terms

- [paper chromatography](https://www.gradenine.co.uk/glossary/paper-chromatography)
- [pure substance](https://www.gradenine.co.uk/glossary/pure-substance)
- [mixture](https://www.gradenine.co.uk/glossary/mixture)
- [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-c929f2cc) · Published by Druglandscape Ltd.