A pharmacist is checking whether a batch of a pain-relief tablet is a pure compound or a mixture. She dissolves a small sample of the tablet in a solvent and carries out paper chromatography. After the chromatography is complete, she observes three separate spots on the paper. She also measures the melting point of the solid tablet and finds that it melts over a range of temperatures rather than at a single sharp temperature. Explain what both pieces of evidence tell the pharmacist about the composition of the tablet.

WJEC A-Level Chemistry (Wales) — 1.7 Equilibria · Explain · 5 marks · View as Markdown

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

Model answer (5 marks)

The three separate spots mean the tablet contains more than one substance – a mixture. A pure compound would give only one spot in paper chromatography. Each spot has a different Rf value, showing the components have different solubilities in the mobile phase and therefore travel different distances. The melting point is not sharp but occurs over a range of temperatures, which is characteristic of a mixture. A pure substance melts at a single, sharp temperature. Together, the chromatography and the melting‑point data confirm that the tablet is a mixture, not a single pure compound.

Examiner tips

  • Mention the number of spots and link to mixture; state Rf differences; explain melting‑point range; combine both pieces of evidence in a concluding sentence

Common mistakes

  • Saying the tablet is pure because it contains several spots; confusing Rf with retention factor; ignoring the melting‑point range or describing it as a single temperature

Mark scheme (5 marks)

  1. The three separate spots indicate the tablet contains more than one substance / is a mixture
  2. A pure substance would produce only a single spot in all solvents, so three spots confirms the sample is not a pure compound
  3. Each spot has a different Rf value, showing the substances in the mixture have different solubilities in the mobile phase / travel different distances
  4. A pure substance melts at a single sharp temperature, so melting over a range of temperatures shows the tablet is not a pure substance / is a mixture
  5. Both pieces of evidence together confirm the tablet is a mixture / formulation, not a single pure compound

Key terms in this question

mixture · melting point · paper chromatography

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