A pharmacist is checking the purity of a white solid painkiller tablet before it is approved for sale. Explain how the pharmacist could use melting point data to determine whether the active ingredient in the tablet is a pure substance or a mixture, and describe what results would be expected in each case.

Eduqas A-Level Chemistry — 1.2 Electronic structure · Explain · 5 marks · View as Markdown

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

The pharmacist has access to a melting point apparatus and a data book listing the exact melting point of the pure active ingredient.

Model answer (5 marks)

A pure substance has a sharp, single melting point. The pharmacist will heat a small sample of the tablet’s active ingredient in the melting point apparatus and record the temperature at which it melts. This measured temperature is then compared with the reference value in the data book. If the measured melting point matches the reference value (within the experimental error) the substance is considered pure.

If the tablet contains a mixture, the sample will melt over a temperature range rather than at one fixed temperature. The pharmacist will observe a broader melting interval and the onset of melting will occur at a temperature lower than the reference value for the pure active ingredient. Thus a lower, broadened melting point indicates a mixture.

Examiner tips

  • Use the phrase "sharp, single melting point" for a pure substance
  • Compare the measured value with the data book value
  • Note that a mixture gives a range and a lower onset temperature

Common mistakes

  • Confusing a lower melting point with a higher one
  • Failing to mention the melting point range for mixtures
  • Not comparing the measured value to the reference value

Mark scheme (5 marks)

  1. A pure substance has a sharp (single/specific) melting point
  2. The measured melting point should be compared to the known melting point of the pure active ingredient (from the data book / reference value)
  3. If the substance is pure, the melting point will match the data book value exactly / be equal to the accepted melting point of the pure active ingredient
  4. A mixture melts over a range of temperatures (rather than at one fixed temperature)
  5. A mixture will also melt at a lower temperature than the pure substance / the melting point of the mixture will be below the data book value

Key terms in this question

pure substance · mixture · melting point data

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