A food scientist is checking the purity of a sample of vanilla flavouring before it is used in a product. She carries out paper chromatography using two solvents. In solvent A, the sample produces a single spot. In solvent B, the sample produces three spots. Explain what these results tell the scientist about the vanilla flavouring sample.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Model answer (4 marks)
A pure compound will give only one spot in every solvent, so the single spot in solvent A shows that the sample is pure in that solvent. The three spots in solvent B mean the sample is not a pure substance – it contains at least three different compounds. The different spots arise because the compounds have different solubilities in the mobile phase and therefore distribute differently between the mobile and stationary phases, travelling different distances.
Examiner tips
- Use the command word ‘explain’ – describe what the results mean, not just state them.
- Mention that a single spot in all solvents indicates purity, while multiple spots indicate a mixture and explain the separation mechanism.
Common mistakes
- Confusing the number of spots with the number of components – a single spot does not prove purity in all solvents.
- Failing to explain why the compounds separate (solubility/distribution differences).
Mark scheme (4 marks)
- A pure compound produces a single spot in all solvents
- The result in solvent B shows the sample is a mixture / is not a pure substance
- The sample contains at least three different compounds / substances
- The compounds separate because they have different solubilities in the mobile phase / travel different distances because they distribute differently between the mobile and stationary phases
Key terms in this question
Related
- All WJEC GCSE Chemistry (Wales) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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