Practise 30 exam-style questions on Chromatography for AQA GCSE Chemistry. Write your answer as you would in the exam and get it marked in seconds against the mark scheme, point by point, with an AI tutor on hand if you get stuck.
30 questions
1–6 marks each
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20 questions, marked as you go
Sample questions with model answers
Here are 4 of the questions in this topic, each with a model answer, an explanation of why it earns the marks and the mark scheme an examiner would use.
Question 1
2 marks
In paper chromatography, name the stationary phase and the mobile phase.
Model answer
The stationary phase is the paper. The mobile phase is the solvent.
Explanation
The stationary phase does not move; the mobile phase moves through it and carries the substances with it. In paper chromatography the solvent soaks up the paper, carrying the dissolved substances different distances.
Mark scheme
1 mark
stationary phase: the (chromatography) paper
1 mark
mobile phase: the solvent
allow a named solvent, e.g. water or ethanol
Question 2
2 marks
When a chromatogram is removed from the solvent, the position of the solvent front must be marked straight away.
Explain why.
Model answer
The solvent evaporates from the paper quickly, so its position soon cannot be seen. The distance moved by the solvent is needed to calculate the Rf values, so the solvent front must be marked in pencil immediately.
Explanation
Without the solvent front, you cannot work out Rf = distance moved by substance ÷ distance moved by solvent. This is also why the paper is removed before the solvent reaches the top.
Mark scheme
1 mark
the solvent evaporates quickly, so the solvent front would soon no longer be visible
1 mark
the distance moved by the solvent is needed to calculate Rf values
Question 3
3 marks
In a paper chromatography experiment, one of the dyes in an ink did not move from the start line.
Explain why. Suggest what the student could do to find out more about this dye.
Model answer
The dye is insoluble in the solvent used, so it does not dissolve in the mobile phase and is not carried up the paper; it stays on the stationary phase at the start line. The student could repeat the chromatography using a different solvent in which the dye is soluble, so that it moves up the paper and its Rf value can be measured.
Explanation
A substance only moves if it dissolves in the mobile phase. Its Rf value in that solvent is 0, which tells you nothing about its identity, so a different solvent is needed.
Mark scheme
1 mark
the dye is insoluble in the solvent
1 mark
so it is not carried by the mobile phase / it stays in the stationary phase (on the paper)
allow 'it is strongly attracted to the paper'
1 mark
repeat the experiment using a different solvent (in which the dye is soluble)
Question 4
4 marks
Explain how paper chromatography separates a mixture of dyes.
Model answer
The solvent is the mobile phase and moves up the paper, which is the stationary phase. Each dye is distributed between the two phases: it dissolves in the solvent but is also attracted to the paper. A dye that is more soluble in the solvent, or less attracted to the paper, spends more of its time in the mobile phase and so travels further up the paper. Different dyes have different solubilities and attractions, so they travel different distances and the mixture separates into separate spots.
Explanation
The key idea is the distribution of each substance between the stationary and mobile phases. A substance that spends more time in the moving solvent travels further and has a higher Rf value.
Mark scheme
1 mark
the solvent (mobile phase) moves up the paper (stationary phase)
1 mark
each dye is distributed between the paper and the solvent / dissolves in the solvent and is attracted to the paper
1 mark
dyes that are more soluble in the solvent / less attracted to the paper spend more time in the mobile phase and travel further
1 mark
different dyes have different solubilities / attractions, so they travel different distances and separate
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