A student is given two white powders. One powder is pure aspirin and the other is a mixture of aspirin and starch. Explain how the student could use melting point data to determine which powder is the pure substance, and what results would be expected for each powder.

Eduqas A-Level Chemistry — 1.5 Energetics · Explain · 5 marks · View as Markdown

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

Aspirin is a pharmaceutical compound with a melting point of 135 °C.

Model answer (5 marks)

Heat both powders in a melting point apparatus and record the temperatures at which they melt.

A pure substance shows a sharp, single melting point.

The pure aspirin will melt sharply at 135 °C.

A mixture will melt over a temperature range.

The aspirin–starch mixture will melt over a range of temperatures, all below 135 °C.

Examiner tips

  • Use the command word ‘Explain’ to describe the procedure and the expected observations; mention sharp vs. range. Include the exact melting point of pure aspirin. Note that the mixture will not reach 135 °C.

Common mistakes

  • Confusing the melting point of the mixture with that of pure aspirin; writing that the mixture melts at 135 °C. Assuming the mixture will melt at a single temperature rather than a range. Omitting the fact that the mixture’s range lies below 135 °C.

Mark scheme (5 marks)

  1. Heat both powders and record the temperature(s) at which they melt
  2. A pure substance has a sharp (single) melting point
  3. The pure aspirin would melt sharply at 135 °C
  4. A mixture melts over a range of temperatures
  5. The aspirin and starch mixture would melt over a range of temperatures (below 135 °C)

Key terms in this question

pure substance · mixture · melting point data

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