# A student has a sample of water collected from a river. The student wants to find out whether the river water is a pure substance or a mixture. Explain how the student could use melting point data to determine whether the river water is pure or impure, and what results would indicate each conclusion.

> Eduqas GCSE Chemistry — 1.3 Water · Explain · 4 marks

## Mark scheme (4 marks)

1. Heat the river water sample and measure/record the temperature at which it melts/freezes (or measure its boiling point / melting point)
2. A pure substance melts/boils at a single, sharp (specific) temperature
3. If the river water is impure (a mixture), it will melt/boil over a range of temperatures rather than at a single temperature
4. Compare the measured melting point to the known melting point of pure water (0 °C) — if it matches sharply, the sample is pure; if it deviates or spans a range, the sample is impure

## Key terms

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

## Related

- [Revision notes for Eduqas GCSE Chemistry](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-has-a-sample-of-42a9f510) · Published by Druglandscape Ltd.