# A pharmacist is checking whether a batch of a pain-relief tablet is a pure compound or a mixture. She dissolves a small sample of the tablet in a solvent and carries out paper chromatography. After the chromatography is complete, she observes three separate spots on the paper. She also measures the melting point of the solid tablet and finds that it melts over a range of temperatures rather than at a single sharp temperature. Explain what both pieces of evidence tell the pharmacist about the composition of the tablet.

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

## Mark scheme (5 marks)

1. The three separate spots indicate the tablet contains more than one substance / is a mixture
2. A pure substance would produce only a single spot in all solvents, so three spots confirms the sample is not a pure compound
3. Each spot has a different Rf value, showing the substances in the mixture have different solubilities in the mobile phase / travel different distances
4. A pure substance melts at a single sharp temperature, so melting over a range of temperatures shows the tablet is not a pure substance / is a mixture
5. Both pieces of evidence together confirm the tablet is a mixture / formulation, not a single pure compound

## Key terms

- [mixture](https://www.gradenine.co.uk/glossary/mixture)
- [melting point](https://www.gradenine.co.uk/glossary/melting-point)
- [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-pharmacist-is-checking-whether-a-00637120) · Published by Druglandscape Ltd.