A chemist is investigating two white solid substances, substance A and substance B. Substance A melts at exactly 58 °C and substance B melts over the range 52 °C to 61 °C. Explain what the melting point data tells the chemist about the purity of each substance.

WJEC A-Level Chemistry (Wales) — 1.6 Kinetics · Explain · 5 marks · View as Markdown

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

Model answer (5 marks)

Substance A has a sharp, single melting point of 58 °C, which indicates it is a pure substance.
Substance B melts over a range from 52 °C to 61 °C, showing it is a mixture or impure. The broad melting range is caused by impurities lowering the melting point and by the different components in the mixture melting at different temperatures.

Examiner tips

  • State the melting point of each substance first. Explain that a single, sharp melting point means purity. Explain that a range indicates impurities or a mixture. Mention that impurities lower the melting point and broaden the range.

Common mistakes

  • Confusing a single melting point with a range. Forgetting to link the range to impurities or a mixture. Using vague terms like "not pure" without explaining why.

Mark scheme (5 marks)

  1. Substance A is a pure substance
  2. Because substance A has a sharp (single/specific) melting point
  3. Substance B is a mixture / impure substance
  4. Because substance B melts over a range of temperatures (rather than at a single fixed temperature)
  5. Impurities lower the melting point and broaden the melting range / the different substances in the mixture melt at different temperatures causing the range

Key terms in this question

melting point data

Related

More Kinetics questions

▶ Try answering this question with AI marking (free) →