A pharmacist is checking the purity of a white powder that should be a single painkiller compound. She measures the melting point of a pure sample of the compound and finds it melts sharply at 153 °C. She then measures the melting point of the powder she has been supplied with and finds it melts over the range 141 °C to 149 °C. Explain what the melting point data tells the pharmacist about the powder she has been supplied with.

Eduqas GCSE Chemistry — 1.2 Atomic structure and the periodic table · Explain · 4 marks · View as Markdown

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

Model answer (4 marks)

The supplied powder is not a pure substance. A pure compound melts at a single, sharp temperature – here 153 °C. The powder melts over a range of 141 °C to 149 °C, showing a broad melting interval. This broad range indicates the powder contains more than one component, i.e. it is a mixture. Therefore the pharmacist cannot use the powder as the correct painkiller and should reject it.

Examiner tips

  • State the difference between a sharp melting point and a range; link the range to impurity.
  • Explain that a mixture shows a broad melting interval.
  • Conclude that the powder is unsuitable and must be rejected.

Common mistakes

  • Confusing a melting point range with a single melting point; writing that the powder is pure.
  • Failing to mention that the broad range indicates a mixture.
  • Not stating that the pharmacist should reject the powder.

Mark scheme (4 marks)

  1. The supplied powder is not a pure substance / is impure
  2. A pure substance has a sharp / specific melting point, but the powder melts over a range of temperatures
  3. Melting over a range of temperatures / the broad melting range indicates the powder is a mixture (not a single compound)
  4. The powder therefore cannot be used / is not the correct substance / the pharmacist should reject / not dispense the powder

Key terms in this question

melting point data

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