A supplier claims that a sample of water is pure. A student measures the melting point of the water sample and finds that it melts over a range of temperatures between −2 °C and 0 °C. Explain what this result tells the student about the water sample.

Eduqas GCSE Chemistry — 1.3 Water · Explain · 4 marks · View as Markdown

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

Model answer (4 marks)

A pure substance has a single, sharp melting point.
The sample melts over a range of temperatures (−2 °C to 0 °C), so it is not pure.
This indicates that at least one other substance is present – the sample is a mixture.
Therefore the supplier’s claim that the water is pure is incorrect.

Examiner tips

  • State the definition of a pure substance’s melting point first. Show the observed range and contrast it with a sharp point. Explain that a range means a mixture. Conclude that the claim is false.

Common mistakes

  • Failing to mention that a pure substance melts at a single temperature. Confusing the melting point with the boiling point. Not linking the range of temperatures to the presence of impurities.

Mark scheme (4 marks)

  1. A pure substance melts at a single/sharp melting point (not over a range)
  2. The sample melts over a range of temperatures, so it is not a pure substance / it is a mixture
  3. The sample contains at least one other substance mixed with the water (i.e. it is a mixture)
  4. Therefore the supplier's claim is incorrect / the sample is not pure water

Key terms in this question

melting point

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