# A food scientist is testing a sports drink to find out whether the orange colouring used in it is a pure substance or a mixture. She carries out paper chromatography on the colouring using water as the solvent. The colouring produces three separate spots on the paper. She then measures the distances travelled and finds that one of the spots has an Rf value of 0.45. Explain what the results of the chromatography tell the scientist about the colouring, and explain how the Rf value could be used to help identify the substance that produced that spot.

> WJEC A-Level Chemistry (Wales) — 1.6 Kinetics · Explain · 5 marks

> A food scientist carries out paper chromatography on an orange colouring taken from a sports drink. She uses water as the solvent. The colouring produces three separate spots on the chromatography paper. One of the spots travels 2.7 cm while the solvent front travels 6.0 cm.

## Mark scheme (5 marks)

1. The colouring is a mixture (not a pure substance)
2. A pure substance would produce only one spot (in all solvents), so three spots shows it contains more than one compound
3. The Rf value is calculated as distance moved by the substance divided by distance moved by the solvent
4. The Rf value of 0.45 can be compared to known / published Rf values of substances (in the same solvent) to identify the compound
5. Different compounds have different Rf values (in the same solvent), so the Rf value is specific to a particular substance

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

## 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-testing-a-efccd702) · Published by Druglandscape Ltd.