A student has two white powders, substance A and substance B. She wants to find out which one is a pure substance and which one is a mixture. She heats samples of each powder and records the temperature at which each one melts. Substance A melts completely at exactly 58 °C. Substance B begins to soften at 49 °C and is fully melted by 61 °C. Explain what the melting point data tells the student about the purity of each substance.

Eduqas GCSE Chemistry — 1.5 Rate of chemical change · Explain · 4 marks · View as Markdown

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

Model answer (4 marks)

Substance A is a pure substance because it has a sharp, single melting point of 58 °C.
Substance B is a mixture because it melts over a range of temperatures, softening at 49 °C and fully melting by 61 °C.

Examiner tips

  • Use the words ‘pure substance’ and ‘mixture’ explicitly. Mention ‘sharp, single melting point’ for purity. Mention ‘range of temperatures’ for mixture. Keep answer to 4 marks, no extra words.

Common mistakes

  • Saying ‘A is pure because it melts’ without noting the single melting point. Calling B pure because it melts, ignoring the temperature range. Using vague terms like ‘clean’ or ‘impure’ instead of ‘pure substance’/‘mixture’.

Mark scheme (4 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
  4. Because substance B melts over a range of temperatures

Key terms in this question

pure substance · mixture · melting point data

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