# A student carries out paper chromatography to investigate a food colouring. The solvent travels 8.0 cm up the paper. One of the spots travels 6.0 cm. The student claims the food colouring is a pure substance because only one spot is visible. Explain why the student's claim may be incorrect, and describe how Rf values can be used to help identify the substances present.

> Eduqas A-Level Chemistry — 1.5 Energetics · Explain · 5 marks

> A food colouring is tested by paper chromatography. The solvent front travels 8.0 cm. A single spot is observed at 6.0 cm.

## Mark scheme (5 marks)

1. A pure substance produces a single spot in all solvents, so one spot in one solvent is not sufficient evidence alone to confirm the substance is pure
2. Two substances with the same Rf value in one solvent could overlap into a single spot, making a mixture appear to be a pure substance
3. The Rf value is calculated as the distance travelled by the substance divided by the distance travelled by the solvent
4. Different compounds have different Rf values in different solvents, so the Rf value can be compared to known values to help identify the substance
5. The more soluble a substance is in the mobile phase (solvent), the further it travels and the higher its Rf value

## Key terms

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

## 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-102d843b) · Published by Druglandscape Ltd.