# A student carries out paper chromatography to investigate two food colourings, X and Y. Food colouring X produces three separate spots on the paper. Food colouring Y produces only one spot. The spot from food colouring Y appears at the same position as one of the spots from food colouring X. Explain what these results tell the student about the two food colourings, and describe how the Rf value of the single spot from food colouring Y could be used to help identify the substance it contains.

> Eduqas A-Level Chemistry — 1.2 Electronic structure · Explain · 5 marks

## Mark scheme (5 marks)

1. Food colouring X is a mixture (because it produces more than one spot)
2. Food colouring Y is a pure substance (because it produces only one spot in this solvent)
3. Food colouring Y contains the same substance as one of the components of food colouring X (because they appear at the same position / same Rf value)
4. Rf value is calculated as the distance travelled by the substance divided by the distance travelled by the solvent
5. The Rf value can be compared to known / reference Rf values (in the same solvent) to identify the substance

## 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 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-carries-out-paper-chromatography-668da3b8) · Published by Druglandscape Ltd.