A student uses paper chromatography to separate the dyes in two fruit drinks, labelled P and Q. The solvent travels 8.0 cm from the baseline. Drink P produces a single spot that travels 6.0 cm. Drink Q produces three spots that travel 2.0 cm, 4.0 cm and 6.0 cm respectively. Using your knowledge of chromatography, explain what these results tell the student about the two drinks.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Model answer (4 marks)
Drink P contains only one dye, so it is a pure substance. A pure compound gives a single spot in paper chromatography.
Drink Q contains more than one dye, so it is a mixture. The different dyes separate because they have different solubilities in the solvent and therefore travel at different speeds, distributing differently between the mobile and stationary phases.
Drink Q contains more than one dye, so it is a mixture. The different dyes separate because they have different solubilities in the solvent and therefore travel at different speeds, distributing differently between the mobile and stationary phases.
Examiner tips
- Use the word 'pure' and 'mixture' to match the scheme.
- Explain the single spot for P and the multiple spots for Q.
- Mention solubility and speed differences for Q.
- Keep each point concise and to the point.
Common mistakes
- Saying 'P is a single component' without linking to purity.
- Forgetting to explain why Q separates (solubility/speed).
- Using vague terms like 'different' without specifying solubility or speed.
Mark scheme (4 marks)
- Drink P is a pure substance / contains only one compound (one dye / one component)
- A pure compound produces a single spot in a solvent
- Drink Q is a mixture / contains more than one compound (dye / component)
- The components in drink Q separate because they have different solubilities in the mobile phase / travel at different speeds in the solvent / are distributed differently between the mobile phase and the stationary phase
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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