A pharmacist is checking the purity of a white powder that should be a single painkiller compound. She measures the melting point of the powder and finds it melts over a range of temperatures between 148 °C and 156 °C. A reference source states that the pure painkiller compound has a sharp melting point of 153 °C. Explain what the melting point data tells the pharmacist about the purity of the powder, and describe one separation technique she could use to obtain a purer sample of the solid compound from its solution.

WJEC A-Level Chemistry (Wales) — 1.1 Atoms, molecules and stoichiometry · Explain · 5 marks · View as Markdown

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

A pharmacist tests a white powder that should contain only one painkiller compound. The melting point of the powder is found to be between 148 °C and 156 °C. The pure compound has a known sharp melting point of 153 °C.

Model answer (5 marks)

A pure substance has a sharp melting point, i.e. it melts at one specific temperature. The powder melts over a range of temperatures (148 °C–156 °C) rather than at a single temperature, so it is not a pure substance – it is a mixture containing impurities.

Because the melting point is depressed and broadened, the powder must contain other substances mixed with the painkiller compound.

To obtain a purer sample the pharmacist could use crystallisation. She would dissolve the powder in a suitable solvent, heat the solution until it becomes saturated and crystals begin to form, then allow the solution to cool slowly so that the painkiller crystallises out. The crystals are collected by filtration and washed to remove remaining impurities.

Examiner tips

  • Mention that a sharp melting point indicates purity; a range indicates impurities. Explain that the range shows the sample is impure. Name crystallisation as the separation technique. Describe heating to saturation, cooling to form crystals, and filtration to collect them.

Common mistakes

  • Saying the powder is pure because it melts somewhere near 153 °C. Forgetting to mention that the range indicates a mixture. Describing a different separation technique such as distillation instead of crystallisation.

Mark scheme (5 marks)

  1. A pure substance has a sharp melting point (melts at one specific temperature)
  2. The powder melts over a range of temperatures, so it is not a pure substance / it is a mixture
  3. The powder therefore contains impurities / other substances mixed with the painkiller compound
  4. Crystallisation should be named as the separation technique
  5. Correct description of crystallisation: heat the solution until crystals start to form / until saturated, allow to cool so crystals form, then filter to collect the crystals

Key terms in this question

melting point data

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