A student uses paper chromatography to investigate whether a food dye is a pure substance or a mixture. The student dissolves the food dye in a solvent and allows the solvent to travel up the paper. Explain how the results of the paper chromatography experiment would allow the student to determine whether the food dye is a pure substance or a mixture.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
A food manufacturer claims that one of its red food dyes is a single pure compound. A student decides to test this claim using paper chromatography.
Model answer (5 marks)
The solvent (mobile phase) moves up the paper (stationary phase). Substances in the dye mixture have different solubilities in the solvent and therefore travel at different rates, giving separate spots. If the dye is a pure compound it will give only one spot. If it is a mixture it will give two or more spots. The Rf value of each spot can be compared with known Rf values to identify the components.
Examiner tips
- Show the separation mechanism (solubility differences).
- State the key observation: one spot = pure, multiple spots = mixture.
- Mention Rf comparison for identification.
Common mistakes
- Confusing mobile and stationary phases.
- Assuming all spots mean pure compound.
- Ignoring the need to compare Rf values with standards.
Mark scheme (5 marks)
- The mobile phase (solvent) carries the substances up the stationary phase (paper)
- Substances in the mixture separate because they have different solubilities in the solvent / travel at different speeds
- If the food dye is a pure substance, it will produce only one spot
- If the food dye is a mixture, it will produce more than one spot
- The Rf value of any spot can be compared to known Rf values to identify the compound(s) present
Key terms in this question
paper chromatography · pure substance · mixture
Related
- All WJEC A-Level Chemistry (Wales) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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