A pharmacist is checking whether a newly manufactured painkiller tablet is a pure substance or a formulation. She measures the melting point of the active ingredient and also carries out paper chromatography on a dissolved sample of the tablet. Explain how the results of both tests could be used to determine whether the tablet contains a single pure substance or a formulation.

Eduqas A-Level Chemistry — 1.1 Atoms, molecules and stoichiometry · Explain · 5 marks · View as Markdown

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

A pharmacist tests a painkiller tablet using melting point apparatus and paper chromatography to check its composition.

Model answer (5 marks)

A pure substance will have a sharp, single melting point, so if the tablet’s active ingredient melts at one specific temperature the tablet is likely pure. If the melting point is broad or shows a range, this indicates a mixture or formulation. In paper chromatography a pure substance gives only one spot; a formulation will give multiple spots. By comparing the Rf values of each spot with known standards the pharmacist can identify the compounds present and confirm whether the tablet contains only the active ingredient or additional excipients.

Examiner tips

  • Mention the sharp single melting point for purity and a broad range for mixtures
  • State that a single spot in chromatography indicates purity, multiple spots indicate a formulation
  • Use Rf comparison to identify components

Common mistakes

  • Confusing a single melting point with a broad range as evidence of purity
  • Assuming any spot in chromatography means impurity without noting that excipients may also appear
  • Failing to mention Rf values for identification

Mark scheme (5 marks)

  1. A pure substance has a sharp (single) melting point / melts at one specific temperature
  2. A formulation / mixture melts over a range of temperatures, so a broad melting range indicates it is not a pure substance
  3. In paper chromatography, a pure substance produces a single spot
  4. A formulation / mixture produces more than one spot because it contains more than one compound
  5. The Rf value of each spot can be compared to known values to identify the substances present in the tablet

Key terms in this question

pure substance · formulation · melting point · paper chromatography

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