# A scientist uses paper chromatography to investigate a sample of black ink taken from a felt-tip pen. The solvent travels 9.6 cm from the baseline. Three separate spots are produced, travelling 2.4 cm, 6.0 cm, and 8.4 cm from the baseline. The scientist concludes that the black ink is not a pure substance. Explain what the results show about the black ink, and describe how Rf values could be used to help identify the individual substances present.

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

## Mark scheme (5 marks)

1. The black ink is a mixture because it produces more than one spot
2. Each spot represents a different substance/compound present in the ink
3. The substances separate because they have different solubilities in the solvent / they are distributed differently between the mobile phase and the stationary phase
4. Rf value is calculated as the distance travelled by the substance divided by the distance travelled by the solvent
5. The Rf values of the spots can be compared to known/reference Rf values in the same solvent to identify each substance

## Key terms

- [paper chromatography](https://www.gradenine.co.uk/glossary/paper-chromatography)
- [Rf value](https://www.gradenine.co.uk/glossary/rf-value)
- [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-scientist-uses-paper-chromatography-to-e2994425) · Published by Druglandscape Ltd.