A student wants to separate the dyes in a sample of black ink using chromatography. Describe how the student would carry out this investigation and explain how the results show that black ink is a mixture of dyes.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Black ink appears to be a single colour, but it is actually made up of several different coloured dyes mixed together. Paper chromatography can be used to separate and identify the components of a mixture.
Model answer (5 marks)
1. Place a small spot of the black ink on the chromatography paper at the baseline, well below the solvent level.
2. Put the paper in a solvent chamber so the solvent rises up the paper.
3. When the solvent front has travelled a suitable distance, remove the paper and let it dry.
4. Observe the chromatogram – the ink separates into several coloured spots at different heights.
5. The different distances travelled are due to the dyes having different solubilities in the solvent and different affinities for the paper, proving that black ink is a mixture of dyes.
2. Put the paper in a solvent chamber so the solvent rises up the paper.
3. When the solvent front has travelled a suitable distance, remove the paper and let it dry.
4. Observe the chromatogram – the ink separates into several coloured spots at different heights.
5. The different distances travelled are due to the dyes having different solubilities in the solvent and different affinities for the paper, proving that black ink is a mixture of dyes.
Examiner tips
- Use the exact sequence of steps to show procedure
- Mention the solvent front and drying
- Explain the role of solubility and affinity
- Show that multiple spots mean a mixture
Common mistakes
- Not specifying the solvent front or drying step
- Failing to explain why different spots appear
- Assuming the ink is a single colour without evidence
Mark scheme (5 marks)
- A spot/line of the black ink is placed near the bottom of the chromatography paper (baseline), above the level of the solvent
- The paper is placed in a suitable solvent so the solvent travels up the paper
- The chromatogram is removed and allowed to dry before results are analysed / the solvent front is marked
- The black ink produces more than one spot/coloured spot at different heights on the chromatogram
- Different dyes travel different distances up the paper because they have different solubilities in the solvent / different affinities for the paper, showing black ink is a mixture
Key terms in this question
Related
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