# A student carries out paper chromatography on a food dye sample to find out whether it is a pure substance or a mixture. The solvent travels 8.0 cm up the paper. One spot produced by the food dye travels 6.0 cm and a second spot travels 2.0 cm. Explain what the results tell the student about the food dye, and state what would be observed if a pure substance were tested instead.

> Eduqas A-Level Chemistry — 1.3 Chemical bonding · Explain · 5 marks

> Paper chromatography separates substances based on how soluble they are in the mobile phase. The Rf value of a substance is calculated as the distance travelled by the dissolved substance divided by the distance travelled by the solvent.

## Mark scheme (5 marks)

1. The food dye produces two spots, showing it is a mixture (not a pure substance) / contains two different substances
2. Correct Rf value calculated for the first spot: 6.0 ÷ 8.0 = 0.75
3. Correct Rf value calculated for the second spot: 2.0 ÷ 8.0 = 0.25
4. The two substances have different Rf values because they have different solubilities in the mobile phase / solvent
5. A pure substance would produce only 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)

## 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)
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Source: [GradeNine](https://www.gradenine.co.uk/q/a-student-carries-out-paper-chromatography-291c4359) · Published by Druglandscape Ltd.