# A student carries out paper chromatography on a food colouring and obtains three separate spots on the paper. Explain what this result tells the student about the food colouring, and describe how the student could use Rf values to help identify the substances present.

> WJEC A-Level Chemistry (Wales) — 1.2 Electronic structure · Explain · 5 marks

> A food manufacturer claims that one of its food colourings is a pure substance. A student dissolves a sample of the food colouring in water and carries out paper chromatography using water as the solvent.

## Mark scheme (5 marks)

1. Three spots means the food colouring is a mixture / is not a pure substance
2. A pure compound would produce only one/a single spot
3. Rf value is calculated as distance travelled by the substance divided by the distance travelled by the solvent
4. Calculate the Rf value for each of the three spots
5. Compare the Rf values obtained to known/reference Rf values (in the same solvent) to identify the substances

## Key terms

- [paper chromatography](https://www.gradenine.co.uk/glossary/paper-chromatography)
- [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-f64f1963) · Published by Druglandscape Ltd.