# A student carries out paper chromatography on a food dye. After the experiment, the solvent has travelled 8.0 cm. The food dye produces only one spot, which has travelled 6.0 cm. Explain what the result tells us about the purity of the food dye, and describe how the Rf value could be used to help identify the substance.

> Eduqas GCSE Chemistry — 1.2 Atomic structure and the periodic table · Explain · 4 marks

> Paper chromatography is used to separate and identify substances in a mixture. A pure compound produces a single spot in all solvents, whereas a mixture separates into different spots.

## Mark scheme (4 marks)

1. The food dye produces only one spot, so it is a pure substance (contains only one compound/element, not mixed with any other substance)
2. The Rf value is calculated as the distance travelled by the substance divided by the distance travelled by the solvent
3. The Rf value for this dye is 0.75
4. The Rf value can be compared to known/reference Rf values (in the same solvent) to identify the substance

## 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 GCSE 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-f180cafc) · Published by Druglandscape Ltd.