A student carries out paper chromatography on a sample of black ink to find out whether it is a pure substance or a mixture. The solvent travels 8.0 cm up the paper. Three separate spots are produced, travelling distances of 2.0 cm, 4.8 cm and 7.2 cm from the baseline. Explain what the results of this experiment tell the student about the black ink, and describe how the mobile phase and stationary phase are involved in producing the separation.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Model answer (4 marks)
The appearance of three distinct spots shows that the black ink is a mixture, not a pure substance, because a pure compound would give only one spot.
The solvent, which is the mobile phase, moves up the paper by capillary action and carries the ink components with it.
The paper itself is the stationary phase; it does not move but adsorbs the components.
The components travel different distances because they have different solubilities in the mobile phase and different affinities for the stationary phase, causing them to separate.
The solvent, which is the mobile phase, moves up the paper by capillary action and carries the ink components with it.
The paper itself is the stationary phase; it does not move but adsorbs the components.
The components travel different distances because they have different solubilities in the mobile phase and different affinities for the stationary phase, causing them to separate.
Examiner tips
- State that multiple spots mean a mixture; a single spot means purity. Explain the roles of mobile and stationary phases. Mention differential solubility/affinity as the separation mechanism.
- common_mistakes
- :
- Saying the solvent is the stationary phase. Claiming the ink is pure because spots are present. Forgetting to link solubility/affinity to the distance travelled.
Mark scheme (4 marks)
- Three spots show the black ink is a mixture (not a pure substance), because a pure substance produces only one spot
- The solvent (mobile phase) carries the substances up the paper as it moves
- The paper acts as the stationary phase and does not move
- The substances separate because they have different solubilities in the mobile phase / they are attracted to the stationary phase to different extents, so they travel different distances
Key terms in this question
paper chromatography · mobile phase · stationary phase · mixture · pure substance
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