A student carries out paper chromatography on a sample of black ink and finds that three separate spots appear on the paper. Explain what this result tells the student about the ink, and describe the role of the mobile phase in separating the substances.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Paper chromatography is used to separate and identify substances in a mixture. The technique relies on a stationary phase and a mobile phase.
Model answer (4 marks)
The result shows that the black ink is a mixture, not a pure substance, because a pure compound would give only one spot. The three spots indicate at least three different components in the ink. The mobile phase, a solvent, moves up the paper by capillary action and carries the ink components with it. The components separate because they have different solubilities in the mobile phase; those that are more soluble travel further, while less soluble ones stay closer to the origin.
Examiner tips
- Mention the mixture vs pure compound point first to secure the 1 mark. Explain the mobile phase’s dual role (carrying and separating) to cover the remaining marks. Use the term "solubility" and "travel different distances" as the mark scheme expects.
- common_mistakes
- :
- Saying the ink is a pure substance. Forgetting that the mobile phase is a solvent. Not linking solubility to the distance travelled.
Mark scheme (4 marks)
- The black ink is a mixture (not a pure substance)
- A pure compound would produce only a single spot
- The mobile phase (solvent) carries the substances through / up the paper
- Substances separate because they have different solubilities in the mobile phase / travel different distances depending on how soluble they are
Key terms in this question
mobile phase · paper chromatography
Related
- All WJEC GCSE Chemistry (Wales) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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