# A student carries out paper chromatography on a sample of food colouring to find out whether it is a pure substance or a mixture. The solvent travels 8.0 cm up the paper. One spot is produced, and it travels 6.0 cm. Explain what the result tells the student about the food colouring, and describe how the Rf value of the spot could be used to help identify the substance present.

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

> Paper chromatography is an analytical technique used to separate and identify substances in a mixture. The Rf value of a substance is calculated as the distance travelled by the dissolved substance divided by the distance travelled by the solvent.

## Mark scheme (5 marks)

1. Only one spot is produced, so the food colouring is a pure substance (or contains only one compound/colour)
2. A mixture would have produced more than one spot
3. Correct calculation of the Rf value: 6.0 ÷ 8.0 = 0.75
4. The Rf value can be compared to known/reference Rf values (for the same solvent) to identify the substance
5. The comparison must be made using the same solvent, as different compounds have different Rf values in different solvents

## Key terms

- [paper chromatography](https://www.gradenine.co.uk/glossary/paper-chromatography)
- [pure substance](https://www.gradenine.co.uk/glossary/pure-substance)
- [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-student-carries-out-paper-chromatography-86c82f48) · Published by Druglandscape Ltd.