# A forensic scientist uses paper chromatography to analyse an ink sample taken from a document. The solvent travels 9.0 cm from the baseline. One of the substances in the ink travels 6.3 cm from the baseline. The scientist says the ink is a mixture, not a pure substance. Explain how paper chromatography works and how the results support the scientist's conclusion that the ink is a mixture.

> WJEC A-Level Chemistry (Wales) — 1.7 Equilibria · Explain · 5 marks

> In paper chromatography, paper acts as the stationary phase and a solvent acts as the mobile phase. The solvent carries substances from the ink up the paper.

## Mark scheme (5 marks)

1. The mobile phase (solvent) moves through/up the stationary phase (paper), carrying the substances with it
2. Substances separate because they have different solubilities in the mobile phase / travel at different speeds
3. A pure compound would produce only a single spot in the solvent
4. The ink produces more than one spot / the components separate into different spots, showing it is a mixture
5. Correct calculation or use of Rf value: distance travelled by substance ÷ distance travelled by solvent = 6.3 ÷ 9.0 = 0.7

## Key terms

- [mixture](https://www.gradenine.co.uk/glossary/mixture)
- [pure substance](https://www.gradenine.co.uk/glossary/pure-substance)

## Related

- [Revision notes for WJEC A-Level Chemistry (Wales)](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-forensic-scientist-uses-paper-chromatography-dbda342d) · Published by Druglandscape Ltd.