8.3 Required practical: chromatography
5 exam questions worth 6 marks. Write your answer, open the mark scheme, then give yourself the marks you earned. Your answers are saved on this device.
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Why is the baseline drawn in pencil?
Choose one answer.
Correct Not quite: the answer is Pencil does not dissolve in the solvent
Mark scheme
A
Model answer
A: pencil (graphite) is insoluble, so it does not move with the solvent.
Explain why the solvent level must be below the baseline.
Mark scheme
- so the spots do not dissolve / wash into the solvent (1)
Model answer
If the solvent were above the line, the spots would dissolve into the solvent in the beaker instead of moving up the paper.
Explain why a lid is put on the beaker.
Mark scheme
- to stop the solvent evaporating (so the paper stays saturated) (1)
Model answer
The lid stops the solvent evaporating from the paper and the beaker.
A food colouring gives one spot in water and one spot in ethanol. What can you conclude?
Mark scheme
- it is probably a pure substance (1)
Model answer
It is probably a single, pure substance, as it gives only one spot in different solvents.
One colouring did not move from the baseline. Suggest why, and what the student should do.
Mark scheme
- it is insoluble in the solvent (1)
- repeat with a different solvent (1)
Model answer
It is insoluble in the solvent used, so the student should repeat the experiment with a different solvent.
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