# A scientist wants to find out whether a food colouring is a pure substance or a mixture of dyes. She uses paper chromatography with two different solvents, Solvent A and Solvent B. In both solvents, the food colouring produces only a single spot. Explain what this result tells the scientist about the food colouring.

> Eduqas GCSE Chemistry — 1.5 Rate of chemical change · Explain · 4 marks

> Paper chromatography separates substances based on how soluble they are in the mobile phase. A pure compound produces a single spot in all solvents, whereas a mixture separates into different spots.

## Mark scheme (4 marks)

1. A single spot in both solvents indicates the food colouring is a pure substance / contains only one compound
2. If it were a mixture, different substances would separate and produce more than one spot
3. Separation in chromatography depends on how soluble each substance is in the mobile phase / solvent
4. Testing in two different solvents strengthens the conclusion / a pure compound produces a single spot in all solvents

## 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)

## 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-scientist-wants-to-find-out-d3273eac) · Published by Druglandscape Ltd.