# A technician uses paper chromatography to check the purity of a sample of a yellow food dye. The solvent front travels 9.0 cm. The yellow dye produces a single spot that travels 6.3 cm. A reference sample of pure yellow dye produces a single spot that travels 6.3 cm. Explain what the technician can conclude about the purity of the sample and describe how Rf values are used to identify the substances present.

> Eduqas A-Level Chemistry — 3.7 Spectroscopy and chromatography · Explain · 5 marks

> Paper chromatography is used in quality control to check whether food dyes are pure. Each substance in a mixture travels a different distance up the chromatography paper, and its identity can be confirmed by calculating its Rf value.

## Mark scheme (5 marks)

1. The sample produces only one spot, so it contains only one substance / is a pure substance / is not a mixture
2. The Rf value of the sample matches the Rf value of the reference / pure yellow dye, confirming the substance is the same
3. Rf value = distance moved by substance ÷ distance moved by solvent front
4. Correct Rf value calculated as 6.3 ÷ 9.0 = 0.70 (allow 0.7)
5. Each substance has a characteristic / unique Rf value (in a given solvent), so Rf values can be compared to known reference values to identify unknowns

## Key terms

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

## Related

- [Revision notes for Eduqas A-Level Chemistry](https://www.gradenine.co.uk/learn)
- [How to answer "Explain" questions](https://www.gradenine.co.uk/tools/command-word-cheatsheet)
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Source: [GradeNine](https://www.gradenine.co.uk/q/a-technician-uses-paper-chromatography-to-6a0e1bc5) · Published by Druglandscape Ltd.