A student tests an unknown sports drink using paper chromatography. The chromatogram shows three spots for the sports drink. The student also runs a sample of pure riboflavin (a food colourant) alongside it, which produces a single spot with an Rf value of 0.45. One of the spots from the sports drink has an Rf value of 0.45 in the same solvent. Explain what the student can conclude about the sports drink from these results.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Model answer (4 marks)
The sports drink is not a pure substance – it gives three spots, so it is a mixture.
One of the spots has an Rf of 0.45, the same as the pure riboflavin reference, so the drink contains riboflavin.
The remaining two spots have different Rf values, indicating two other components that are not riboflavin.
The comparison is only valid because the same solvent system was used for both samples, as Rf values depend on the solvent.
One of the spots has an Rf of 0.45, the same as the pure riboflavin reference, so the drink contains riboflavin.
The remaining two spots have different Rf values, indicating two other components that are not riboflavin.
The comparison is only valid because the same solvent system was used for both samples, as Rf values depend on the solvent.
Examiner tips
- State the mixture conclusion first, then the identification of riboflavin, then the other components, and finally note the solvent requirement.
- Use the exact phrase ‘Rf value of 0.45’ and mention the solvent system to show understanding of Rf dependence.
Common mistakes
- Confusing the number of spots with purity, or claiming the drink is pure.
- Ignoring the need for the same solvent system when comparing Rf values.
Mark scheme (4 marks)
- The sports drink is a mixture (not a pure substance) because it produces more than one spot
- The sports drink contains riboflavin because one spot has the same Rf value (0.45) as pure riboflavin
- The other two spots represent different compounds / components that are not riboflavin
- Rf values depend on the solvent used, so the same solvent must be used for a valid comparison / the identification is only valid in this solvent
Key terms in this question
Related
- All AQA GCSE Chemistry (8462) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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