# A student uses paper chromatography to analyse a mixture of food colourings. The student calculates the Rf value for one of the food colourings and finds it is identical to the Rf value of a known pure dye. Explain what Rf values are, how they are calculated, and what the student can conclude from this result.

> OCR A-Level Chemistry A (H432) — 6.3 Analysis (chromatography, NMR) · Explain · 5 marks

> Paper chromatography is a technique used to separate and identify substances in a mixture. Each substance travels a characteristic distance up the chromatography paper relative to the solvent front.

## Mark scheme (5 marks)

1. The Rf value is the ratio of the distance travelled by the substance to the distance travelled by the solvent front
2. The Rf value is always between 0 and 1
3. Each pure substance has a unique/characteristic Rf value (in a given solvent)
4. Because the Rf value of the food colouring matches the Rf value of the known pure dye, the food colouring may be (or is likely to contain) the same substance as the known pure dye
5. This result alone cannot conclusively prove the substances are identical; a different solvent or additional test would be needed to confirm identity

## Key terms

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

## Related

- [Revision notes for OCR A-Level Chemistry A (H432)](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-student-uses-paper-chromatography-to-c17d2c19) · Published by Druglandscape Ltd.