A pharmacist is checking the purity of a batch of paracetamol tablets before they are sold to the public. She records the melting point of a sample taken from the batch and compares it with the known melting point of pure paracetamol, which is 169 °C. The sample begins to melt at 162 °C and finishes melting at 171 °C. Explain what the melting point data tells the pharmacist about the purity of the sample, and explain why physical separation processes rather than chemical processes are used to purify substances.

WJEC A-Level Chemistry (Wales) — 1.4 States of matter · Explain · 5 marks · View as Markdown

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

Pure paracetamol has a sharp melting point of 169 °C. The sample from the batch begins to melt at 162 °C and is fully melted at 171 °C.

Model answer (5 marks)

The sample does not melt at a single sharp temperature but over a range from 162 °C to 171 °C. A pure substance has a single, sharp melting point, so the broad range shows the sample is not pure – it contains impurities that lower and broaden the melting point. The fact that melting starts below the known 169 °C is further evidence of impurities.

Physical separation processes are used because they do not involve chemical reactions; the chemical structure of the substance remains unchanged. This allows the desired compound to be isolated without altering or destroying it.

Examiner tips

  • State that a pure substance has a sharp melting point; note the range indicates impurity.
  • Explain that impurities lower and broaden the melting point. Mention that physical methods keep the chemical properties intact.

Common mistakes

  • Saying the sample is pure because it melts at 169 °C – ignores the range. Forgetting to explain that physical separation does not change the chemical structure.

Mark scheme (5 marks)

  1. The sample melts over a range of temperatures (not at a single sharp point), indicating it is not a pure substance / is impure / is a mixture
  2. A pure substance has a sharp (single / specific) melting point
  3. The sample starts melting below 169 °C, which is further evidence that impurities are present (impurities lower and broaden the melting point)
  4. Physical separation processes do not involve chemical reactions
  5. So the chemical properties of the substance being purified remain unchanged / the desired substance is not altered or destroyed

Key terms in this question

melting point data

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