A student has two white powder samples. One sample is a pure substance and the other is a mixture. Describe how the student could use melting point data to determine which sample is the pure substance, and explain what results would be expected for each sample.

Eduqas A-Level Chemistry — 1.4 States of matter · Describe · 5 marks · View as Markdown

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

Model answer (5 marks)

Heat both samples in a melting point apparatus and record the temperature(s) at which each sample begins to melt.

The sample that shows a single, sharp melting point (a narrow temperature range, e.g. 1–2 °C) is the pure substance.

The other sample will melt over a broader temperature range, indicating it is a mixture.

The sharp melting point can then be compared with a standard value to confirm the identity of the pure substance.

Examiner tips

  • Use the phrase ‘sharp melting point’ and ‘melting over a range’ – these are key terms in the mark scheme.
  • Mention that the pure sample’s melting point should be compared with a known value to confirm identity.

Common mistakes

  • Failing to state that the pure sample has a single, sharp melting point.
  • Confusing the melting point of the mixture with that of the pure substance, or not recognising the range of temperatures for the mixture.

Mark scheme (5 marks)

  1. Heat both samples and record the temperature(s) at which each sample melts
  2. The pure substance will have a sharp (single, specific) melting point
  3. The mixture will melt over a range of temperatures
  4. The sample that shows a sharp melting point is identified as the pure substance
  5. The recorded melting point can be compared to a known/data book value to confirm the identity of the pure substance

Key terms in this question

pure substance · mixture · melting point data

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