A student is given two white solid substances, substance A and substance B. The student is told that one of these substances is a pure substance and the other is a mixture. Describe how the student could use melting point data to identify which substance is the pure substance and which is the mixture. In your answer, state the result expected for each substance.

WJEC A-Level Chemistry (Wales) — 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)

1. Measure the melting point of each substance using a suitable method.
2. Record the temperature at which each substance starts to melt and when it is completely liquid.
3. If a substance melts at a single, sharp temperature (e.g. 180 °C to 180.5 °C) it is a pure substance.
4. If a substance melts over a range of temperatures (e.g. 120 °C to 140 °C) it is a mixture.
5. Therefore, the substance that shows a sharp melting point is the pure substance, and the one that melts over a range is the mixture.

Examiner tips

  • Use the word ‘measure’ and mention a suitable method (e.g. melting point apparatus).
  • Show the difference between a sharp melting point and a range – this is the key criterion.
  • State which substance is pure and which is a mixture explicitly.

Common mistakes

  • Failing to mention the method of measuring the melting point.
  • Confusing the terms ‘sharp’ and ‘range’ or not recognising a range as evidence of a mixture.
  • Not specifying which substance (A or B) is pure and which is a mixture.

Mark scheme (5 marks)

  1. Heat/measure the melting point of each substance (using a suitable method)
  2. A pure substance melts at a specific/sharp temperature
  3. A mixture melts over a range of temperatures
  4. The substance (A or B) that shows a sharp melting point is the pure substance
  5. The substance (A or B) that melts over a range of temperatures is the mixture

Key terms in this question

melting point · pure substance · mixture

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