# A pharmacist is checking the purity of a batch of paracetamol tablets before they are sold to the public. She records the melting point of a sample taken from the batch and compares it with the known melting point of pure paracetamol, which is 169 °C. The sample begins to melt at 162 °C and finishes melting at 171 °C. Explain what the melting point data tells the pharmacist about the purity of the sample, and explain why physical separation processes rather than chemical processes are used to purify substances.

> WJEC A-Level Chemistry (Wales) — 1.4 States of matter · Explain · 5 marks

> Pure paracetamol has a sharp melting point of 169 °C. The sample from the batch begins to melt at 162 °C and is fully melted at 171 °C.

## Mark scheme (5 marks)

1. The sample melts over a range of temperatures (not at a single sharp point), indicating it is not a pure substance / is impure / is a mixture
2. A pure substance has a sharp (single / specific) melting point
3. The sample starts melting below 169 °C, which is further evidence that impurities are present (impurities lower and broaden the melting point)
4. Physical separation processes do not involve chemical reactions
5. So the chemical properties of the substance being purified remain unchanged / the desired substance is not altered or destroyed

## Key terms

- [melting point data](https://www.gradenine.co.uk/glossary/melting-point-data)

## 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-the-purity-256ad6a0) · Published by Druglandscape Ltd.