A pharmacist is checking the purity of a painkiller tablet. She dissolves a small amount of the tablet in a solvent and carries out paper chromatography alongside a sample of the pure active ingredient. The painkiller sample produces three separate spots on the chromatogram, whilst the pure active ingredient produces only one spot. Explain what these results tell the pharmacist about the painkiller tablet, and explain how paper chromatography causes substances in a mixture to separate.

WJEC GCSE Chemistry (Wales) — 1.4 The ever-changing Earth · Explain · 4 marks · View as Markdown

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

Model answer (4 marks)

The painkiller tablet is not a pure substance – it is a mixture. The three spots on the chromatogram mean that three different compounds are present in the tablet. In paper chromatography the solvent (mobile phase) moves up the paper (stationary phase). Substances are carried upward by the solvent but travel at different rates because they have different solubilities in the mobile phase – the more soluble a compound, the further it travels.

Examiner tips

  • Use the word ‘mixture’ to show the tablet is not pure.
  • Mention the three spots as evidence of three different compounds.
  • Explain the roles of mobile and stationary phases.
  • State that separation is due to differing solubilities in the solvent.”]

Mark scheme (4 marks)

  1. The painkiller tablet is a mixture / is not a pure substance
  2. The three spots indicate three different compounds / substances are present in the tablet
  3. In paper chromatography, the mobile phase (solvent) carries the substances up the stationary phase (paper)
  4. Substances separate because they have different solubilities in the mobile phase / more soluble substances travel further up the paper

Key terms in this question

paper chromatography · mixture

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