A student uses paper chromatography to separate the dyes present in a sample of plant extract. The student runs the chromatogram using two different solvents. In the first solvent, three spots are visible. In the second solvent, four spots are visible. Explain what the student can conclude about the composition of the plant extract and how paper chromatography separates the different dyes.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Paper chromatography can be used to separate and identify the components of a mixture. Each component travels a different distance up the chromatography paper depending on how it interacts with the stationary phase and the mobile phase.
Model answer (5 marks)
The plant extract contains at least four different dyes. The first solvent shows three spots, so it separates three components. The second solvent shows four spots, meaning it separates one more component or separates a component that was previously overlapping. Therefore the extract must contain at least four dyes. Paper chromatography separates dyes by the relative affinity of each dye for the stationary phase (paper) versus the mobile phase (solvent). The solvent carries the dyes up the paper. A dye that is more soluble in the solvent or interacts more strongly with the mobile phase travels further up the paper. A dye that interacts more strongly with the stationary phase is retained and travels a shorter distance.
Examiner tips
- Use the word ‘separates’ to show understanding of the technique; mention mobile and stationary phases; explain why the second solvent reveals more spots; state the minimum number of components; keep answer concise and use UK spelling.
Common mistakes
- Saying the extract contains exactly four dyes; confusing mobile phase with stationary phase; not explaining why the second solvent separates more components.
Mark scheme (5 marks)
- The plant extract contains a mixture of (at least four) different dyes / components
- The second solvent separates the components more effectively / reveals more components, so the number of components is at least four
- The mobile phase (solvent) carries the dyes up the paper
- Dyes that are more soluble in the solvent / interact more strongly with the mobile phase travel further up the paper
- Dyes that interact more strongly with the stationary phase (paper) travel less far / are retained more
Key terms in this question
Related
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