# A student carries out paper chromatography to investigate an unknown food dye. The solvent travels 8.0 cm from the baseline. One spot produced by the unknown dye travels 6.0 cm from the baseline. A known pure dye X has a recorded Rf value of 0.55 in the same solvent. Explain how the student can use the results of the chromatography to determine whether the unknown dye is a pure substance and whether it is the same as dye X.

> WJEC A-Level Chemistry (Wales) — 1.7 Equilibria · Explain · 5 marks

## Mark scheme (5 marks)

1. A pure substance produces only one spot in a solvent / a mixture produces more than one spot
2. Rf value = distance travelled by substance ÷ distance travelled by solvent
3. Correct calculation of Rf: 6.0 ÷ 8.0 = 0.75
4. The Rf value of the unknown spot (0.75) does not match the Rf value of dye X (0.55), so the unknown dye is not the same as dye X
5. Different compounds have different Rf values in the same solvent, so Rf values can be used to identify or distinguish substances

## 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 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-74a8dcc3) · Published by Druglandscape Ltd.