# A food scientist is investigating whether a sample of a yellow food colouring is a pure substance or a mixture. The scientist uses paper chromatography, with ethanol as the solvent, and obtains three separate spots on the paper. Explain what the results tell the scientist about the food colouring, and describe how the Rf value of one of the spots could be used to help identify the substance responsible for that spot.

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

> A food scientist carries out paper chromatography on a yellow food colouring using ethanol as the solvent. The chromatogram produces three separate spots at different positions on the paper. The solvent travels 12.0 cm from the baseline. One of the spots travels 9.0 cm from the baseline.

## Mark scheme (5 marks)

1. Three spots show the food colouring is a mixture (not a pure substance)
2. The three substances in the mixture have different solubilities in the mobile phase (ethanol), causing them to separate
3. The Rf value is calculated as the distance travelled by the substance divided by the distance travelled by the solvent
4. Correct Rf value calculated for the spot that travels 9.0 cm: Rf = 9.0 ÷ 12.0 = 0.75
5. The Rf value can be compared to known/reference Rf values of substances 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)
- [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-food-scientist-is-investigating-whether-b06150ef) · Published by Druglandscape Ltd.