A student wants to find out whether a sample of green ink is a pure substance or a mixture. She carries out paper chromatography using two different solvents. In the first solvent, the ink produces a single spot. In the second solvent, the ink produces three separate spots. Explain what conclusions the student can draw from these results, and explain how the mobile phase and stationary phase are involved in the separation process.
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 coloured substances. A pure compound produces a single spot in all solvents, whereas a mixture separates into different spots.
Model answer (5 marks)
The single spot obtained with the first solvent does not prove that the ink is pure, because a single spot is only conclusive when all solvents give a single spot. The appearance of three distinct spots with the second solvent shows that the ink contains at least three different substances, i.e. it is a mixture.
In paper chromatography the mobile phase is the solvent that moves up the paper by capillary action. The stationary phase is the paper itself, which remains fixed. Separation occurs because the different components of the mixture have different affinities for the mobile phase and the stationary phase; they partition differently between the two, so they travel different distances up the paper.
In paper chromatography the mobile phase is the solvent that moves up the paper by capillary action. The stationary phase is the paper itself, which remains fixed. Separation occurs because the different components of the mixture have different affinities for the mobile phase and the stationary phase; they partition differently between the two, so they travel different distances up the paper.
Examiner tips
- Mention that a single spot in one solvent is not conclusive; all solvents must give one spot to claim purity.
- State clearly that three spots mean a mixture of at least three components.
- Identify the mobile phase as the solvent and the stationary phase as the paper.
- Explain that separation is due to differing solubilities/affinities between mobile and stationary phases.
Common mistakes
- Claiming that a single spot in one solvent proves purity.
- Confusing the mobile phase with the stationary phase.
- Failing to mention that the second solvent shows a mixture.
Mark scheme (5 marks)
- The single spot in the first solvent does NOT confirm the ink is a pure substance (because a single spot is only conclusive when all solvents give one spot)
- The three separate spots in the second solvent show the ink is a mixture (containing at least three different compounds/substances)
- The mobile phase is the solvent, which moves through the chromatography system carrying the substances in the mixture
- The stationary phase is the paper, which does not move
- Separation occurs because the different substances in the mixture have different solubilities in the mobile phase / distribute differently between the mobile and stationary phases, so they travel different distances up the paper
Key terms in this question
mobile phase · stationary phase · pure substance · mixture · paper chromatography
Related
- All WJEC A-Level Chemistry (Wales) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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