A student uses paper chromatography to analyse a sample of food colouring. The chromatography is run using water as the solvent. The student then runs the same food colouring sample using ethanol as the solvent. Explain how paper chromatography separates the components of a mixture, and suggest why the two solvents may produce different results for the same food colouring sample.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Paper chromatography is a technique used to separate and identify the different components in a mixture. A sample is placed on the baseline of chromatography paper and a solvent is allowed to travel up the paper. Different components travel different distances up the paper.
Model answer (5 marks)
Paper chromatography separates a mixture by exploiting differences in solubility and affinity for the stationary phase. The solvent rises by capillary action, carrying the mixture components with it. Components that are more soluble in the solvent, or are more attracted to the solvent, travel further up the paper. Components that are more strongly attracted to or held by the paper (the stationary phase) travel a shorter distance. Thus each component has a characteristic Rf value.
Using water as the solvent, the food‑colouring components will move according to their solubility in water and their interaction with the paper. Switching to ethanol changes the solvent’s polarity and its ability to dissolve the components. Ethanol may dissolve some components more readily or reduce their affinity for the paper, giving different Rf values. Consequently, the two solvents can separate the same mixture into different numbers of spots or reveal components that were co‑moved with water.
Using water as the solvent, the food‑colouring components will move according to their solubility in water and their interaction with the paper. Switching to ethanol changes the solvent’s polarity and its ability to dissolve the components. Ethanol may dissolve some components more readily or reduce their affinity for the paper, giving different Rf values. Consequently, the two solvents can separate the same mixture into different numbers of spots or reveal components that were co‑moved with water.
Examiner tips
- Explain the role of solvent, stationary phase and solubility; mention Rf values; justify why a different solvent changes separation
Common mistakes
- Confusing solvent with stationary phase; not mentioning Rf; claiming only one component is separated
Mark scheme (5 marks)
- The solvent moves up the chromatography paper carrying the components of the mixture with it
- Components that are more soluble in the solvent / attracted more to the solvent travel further up the paper
- Components that are more attracted to / more strongly held by the paper (stationary phase) travel less far
- Different components have different solubilities in different solvents, so the Rf values / distances travelled will differ between water and ethanol
- A different solvent may be able to separate components that appeared as one spot with the other solvent / may reveal components not separated by water
Key terms in this question
Related
- All OCR A-Level Chemistry B: Salters (H433) revision notes →
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