A pharmaceutical company makes two products: pure aspirin tablets and a cold-relief formulation. Explain why only the pure aspirin tablets would show a sharp melting point, and describe how paper chromatography could be used to confirm that the aspirin used in the cold-relief formulation is mixed with other substances.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Model answer (5 marks)
A pure substance melts at a single, sharp temperature because all its molecules are identical and have the same lattice energy. The pure aspirin tablets therefore show a sharp melting point.
The cold‑relief formulation is a mixture of aspirin with other excipients. In a mixture the different components melt over a range of temperatures, so the formulation does not give a single sharp melting point.
Paper chromatography can confirm the presence of impurities. The solvent (mobile phase) carries the mixture up the paper (stationary phase). Different substances have different affinities for the paper and the solvent, so they travel different distances and appear as separate spots. If more than one spot is observed, this proves that the aspirin in the cold‑relief formulation is mixed with other substances and is not pure.
The cold‑relief formulation is a mixture of aspirin with other excipients. In a mixture the different components melt over a range of temperatures, so the formulation does not give a single sharp melting point.
Paper chromatography can confirm the presence of impurities. The solvent (mobile phase) carries the mixture up the paper (stationary phase). Different substances have different affinities for the paper and the solvent, so they travel different distances and appear as separate spots. If more than one spot is observed, this proves that the aspirin in the cold‑relief formulation is mixed with other substances and is not pure.
Examiner tips
- Use the term ‘sharp melting point’ for pure aspirin; mention ‘range of temperatures’ for the mixture. Explain that paper chromatography separates components by differing Rf values. Show that multiple spots = mixture, single spot = pure.
Mark scheme (5 marks)
- A pure substance melts at one specific/sharp temperature (not over a range)
- The cold-relief formulation is a mixture, so it melts/boils over a range of temperatures (not at a single sharp point)
- A formulation is a mixture designed for a particular use, made up of exact quantities of pure substances (accept: the cold-relief product contains more than one substance mixed together)
- In paper chromatography, the solvent (mobile phase) carries the substances up the paper (stationary phase) and different substances travel different distances / separate into different spots
- If more than one spot is produced, this confirms the aspirin sample is mixed with other substances / is not pure (a pure compound produces only one spot)
Key terms in this question
formulation · paper chromatography
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