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 · View as Markdown

Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).

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

Model answer (4 marks)

A pure substance melts at a single, sharp temperature.
A mixture melts over a range of temperatures.
If the river water is pure, it will melt at exactly 0 °C.
If it is impure, the melting point will be lower than 0 °C or will occur over a temperature range.

Examiner tips

  • Use the command word ‘describe’ – give clear, concise points. Show the difference between a sharp melting point and a range. Mention the exact temperature for pure water (0 °C).

Common mistakes

  • Confusing boiling point with melting point. Saying the mixture melts at a higher temperature instead of lower. Omitting the sharp‑point vs. range distinction.

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 in this question

pure substance · mixture · melting point

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