# A student uses paper chromatography to separate the coloured pigments in a sample of black ink. After running the chromatogram, five separate spots are visible. The solvent front has travelled 9.0 cm. One spot has travelled 6.3 cm and another has travelled 1.8 cm. Explain how the student could use the chromatogram to identify whether any of the pigments in the black ink are pure substances, and describe how Rf values are calculated and used in this identification.

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

> Paper chromatography is used to separate mixtures. Each component in a mixture can be identified by calculating its Rf value and comparing it to known reference values.

## Mark scheme (5 marks)

1. Rf value is calculated by dividing the distance travelled by the substance (spot) by the distance travelled by the solvent front
2. Correct calculation of at least one Rf value — 6.3 ÷ 9.0 = 0.70 or 1.8 ÷ 9.0 = 0.20
3. The calculated Rf value is compared to Rf values of known, pure substances (reference/standard values)
4. A pure substance produces only one spot / gives a single spot on the chromatogram (does not separate further)
5. Since five spots are visible, the black ink is a mixture of at least five different substances / is not a pure substance

## Key terms

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

## 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)
- [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-0293441e) · Published by Druglandscape Ltd.