A chemist has produced a sample of an unknown solid compound and wants to check whether it is pure. Explain how melting point data can be used to evaluate the purity of the solid, and describe what results the chemist would expect to see for both a pure substance and an impure substance.

Edexcel GCSE Chemistry (1CH0) — 2.2 Methods of separating and purifying substances · 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 sharp, specific melting point – it melts at one precise temperature. An impure substance melts over a range of temperatures, not at a single point. The presence of impurities usually lowers the melting point of the solid. The chemist can therefore compare the measured melting point to the literature value: if the temperature matches the known value and the transition is sharp, the sample is pure; if the temperature is lower and the transition is broad, the sample is impure.

Examiner tips

  • State that a pure compound melts at one temperature; an impure one melts over a range.
  • Mention that impurities lower the melting point.
  • Show the comparison with the literature value to confirm identity and purity.
  • Use the exact terms "sharp" and "specific" for a pure substance.”]

Mark scheme (4 marks)

  1. A pure substance has a sharp/specific melting point (melts at one precise temperature)
  2. An impure substance melts over a range of temperatures (not at a single point)
  3. An impure substance has a lower melting point than the pure substance / impurities lower the melting point
  4. The chemist can compare the measured melting point to a known/literature value to confirm the identity AND purity of the substance

Key terms in this question

melting point data · pure substance

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