# A scientist wants to find out whether a sample of water collected from a river is a pure substance or a mixture. Describe how the scientist could use melting point data to determine this, and explain what results would indicate that the sample is pure water.

> AQA GCSE Chemistry (8462) — 4.8.1 Purity, formulations and chromatography · Describe · 4 marks

> Pure water melts at 0 °C. The scientist measures the melting point of the frozen river water sample.

## Mark scheme (4 marks)

1. A pure substance melts at a specific / sharp melting point
2. A mixture melts over a range of temperatures (rather than at one fixed point)
3. If the sample is pure water, it would melt at exactly 0 °C
4. If the sample is a mixture / impure, its melting point would differ from / be below 0 °C

## Key terms

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

## Related

- [Revision notes for AQA GCSE Chemistry (8462)](https://www.gradenine.co.uk/learn)
- [How to answer "Describe" questions](https://www.gradenine.co.uk/tools/command-word-cheatsheet)
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Source: [GradeNine](https://www.gradenine.co.uk/q/a-scientist-wants-to-find-out-27e8cbad) · Published by Druglandscape Ltd.