# Paper chromatography can be used to separate the pigments in a food colouring. A student runs paper chromatography on a green food colouring and on two pure dyes, dye X and dye Y, using the same solvent. The green food colouring produces two spots. One spot travels 4.8 cm and the other travels 2.4 cm. The solvent front travels 6.0 cm. Dye X has an Rf value of 0.80 and dye Y has an Rf value of 0.30. Explain what the results tell the student about the green food colouring and identify the dyes it contains.

> WJEC GCSE Chemistry (Wales) — 1.3 Water · Explain · 4 marks

> The student uses the same solvent for all three samples. The solvent front travels 6.0 cm in each case. The green food colouring produces two spots: one travelling 4.8 cm and one travelling 2.4 cm.

## Mark scheme (4 marks)

1. The green food colouring is a mixture (not a pure substance) because it produces more than one spot.
2. Rf value of the spot travelling 4.8 cm is 0.80 (4.8 ÷ 6.0 = 0.80).
3. The spot with Rf 0.80 matches dye X, so the food colouring contains dye X.
4. The spot travelling 2.4 cm has an Rf value of 0.40, which does not match dye X or dye Y, so the second pigment is unknown / cannot be identified from the data given. OR: Rf of the second spot is 0.40 and does not match either pure dye.

## Key terms

- [Rf value](https://www.gradenine.co.uk/glossary/rf-value)
- [paper chromatography](https://www.gradenine.co.uk/glossary/paper-chromatography)

## Related

- [Revision notes for WJEC GCSE Chemistry (Wales)](https://www.gradenine.co.uk/learn)
- [How to answer "Explain" questions](https://www.gradenine.co.uk/tools/command-word-cheatsheet)
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