# A student wants to find out whether a sample of green ink is a pure substance or a mixture. She carries out paper chromatography using two different solvents. In the first solvent, the ink produces a single spot. In the second solvent, the ink produces three separate spots. Explain what conclusions the student can draw from these results, and explain how the mobile phase and stationary phase are involved in the separation process.

> WJEC A-Level Chemistry (Wales) — 1.6 Kinetics · Explain · 5 marks

> Paper chromatography can be used to separate and identify the components of coloured substances. A pure compound produces a single spot in all solvents, whereas a mixture separates into different spots.

## Mark scheme (5 marks)

1. The single spot in the first solvent does NOT confirm the ink is a pure substance (because a single spot is only conclusive when all solvents give one spot)
2. The three separate spots in the second solvent show the ink is a mixture (containing at least three different compounds/substances)
3. The mobile phase is the solvent, which moves through the chromatography system carrying the substances in the mixture
4. The stationary phase is the paper, which does not move
5. Separation occurs because the different substances in the mixture have different solubilities in the mobile phase / distribute differently between the mobile and stationary phases, so they travel different distances up the paper

## Key terms

- [mobile phase](https://www.gradenine.co.uk/glossary/mobile-phase)
- [stationary phase](https://www.gradenine.co.uk/glossary/stationary-phase)
- [pure substance](https://www.gradenine.co.uk/glossary/pure-substance)
- [mixture](https://www.gradenine.co.uk/glossary/mixture)
- [paper chromatography](https://www.gradenine.co.uk/glossary/paper-chromatography)

## 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-wants-to-find-out-5294ffaf) · Published by Druglandscape Ltd.