# A student carries out paper chromatography to investigate two food colourings, X and Y. Food colouring X produces a single spot. Food colouring Y produces three spots. The student measures the distances travelled by one of the spots in Y and by the solvent front. Using this information, explain what conclusions the student can draw about food colourings X and Y, and describe how the student could use the Rf value to help identify the substance that produced the spot measured in Y.

> Eduqas A-Level Chemistry — 1.7 Equilibria · Explain · 5 marks

> Food colouring X produces a single spot on the chromatogram. Food colouring Y produces three spots. The solvent travels 8.0 cm from the baseline. One spot in food colouring Y travels 4.0 cm from the baseline.

## Mark scheme (5 marks)

1. Food colouring X is a pure substance / contains only one compound because it produces a single spot
2. Food colouring Y is a mixture because it produces more than one spot / separates into three spots
3. The substances in Y separate because they have different solubilities in the mobile phase / solvent
4. Rf value = distance travelled by the substance ÷ distance travelled by the solvent
5. The calculated Rf value can be compared to known / data-book Rf values to identify the substance

## Key terms

- [Rf value](https://www.gradenine.co.uk/glossary/rf-value)
- [paper chromatography](https://www.gradenine.co.uk/glossary/paper-chromatography)

## 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-b1087d50) · Published by Druglandscape Ltd.