A student carries out paper chromatography to investigate whether a food dye is a pure substance or a mixture. The food dye produces three separate spots on the paper. Explain what this result tells the student about the food dye, and describe how the student could use Rf values to identify each substance present.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Model answer (5 marks)
Three spots mean the dye is a mixture, not a pure compound (a pure substance would give only one spot in any solvent). The Rf value is calculated as the distance travelled by the spot divided by the distance travelled by the solvent front. Each spot has a different Rf value because each component moves a different distance. By comparing these Rf values with reference values for known dyes, the student can identify each component of the mixture.
Examiner tips
- State that three spots = mixture; one spot = pure. Define Rf = distance of spot / distance of solvent front. Note each spot has a distinct Rf. Explain comparison with reference Rf values to identify substances.
Common mistakes
- Confusing Rf with percentage of solvent front. Assuming all spots have the same Rf. Failing to mention that a pure compound gives only one spot.
Mark scheme (5 marks)
- Three spots indicate the food dye is a mixture (not a pure substance)
- A pure compound would produce only a single spot (in all solvents)
- The Rf value is calculated as the distance travelled by the substance divided by the distance travelled by the solvent
- Each spot gives a different Rf value (because each substance travels a different distance)
- The Rf values can be compared to known/reference Rf values to identify each substance in the mixture
Key terms in this question
paper chromatography · pure substance · mixture · Rf value
Related
- All Eduqas A-Level Chemistry revision notes →
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- Decode the mark scheme abbreviations →
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