A student tests two white solid substances, substance A and substance B, to find out which one is a pure substance. Substance A melts completely at 58 °C. Substance B begins to melt at 54 °C and is fully melted by 61 °C. Explain what the melting point data tells the student about the purity of each substance.

WJEC GCSE Chemistry (Wales) — 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)

Substance A has a sharp melting point – it melts completely at 58 °C, showing a single, well‑defined transition.

A sharp melting point indicates that the sample is pure.

Substance B melts over a range of temperatures – it starts at 54 °C and is fully melted by 61 °C, showing a broad transition.

A broad melting range indicates the sample is impure or a mixture.

Examiner tips

  • Use the term "sharp melting point" for pure substances and "broad/melting range" for impure ones.
  • Show the temperature data for each substance before stating the conclusion.
  • Link the melting point behaviour directly to purity.
  • Keep the answer concise – 4 marks only.

Common mistakes

  • Confusing a single melting point with a range; writing that a pure substance melts over a range.
  • Failing to mention the temperature values or the specific temperatures given in the question.

Mark scheme (4 marks)

  1. Substance A has a sharp melting point (melts at one specific temperature)
  2. Therefore substance A is a pure substance
  3. Substance B melts over a range of temperatures
  4. Therefore substance B is impure / a mixture

Key terms in this question

pure substance · melting point data

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