AQA GCSE Chemistry (8462), Higher tier · Chemical analysis › Purity, formulations and chromatography
Practise Chromatography. 19 exam-style questions plus unlimited generated ones on this subtopic, at up to four difficulty levels, with full mark schemes and a progress tracker. Free, no account needed.
Required practical:Chromatography (method, variables and exam tips)
Quick recall
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A pure substance is tested using three different solvents. How many spots would you expect on each chromatogram?
1
A student wants to find out whether the black ink in a pen is a mixture and whether it contains any of three known dyes, J, K and L. What is the stationary phase in paper chromatography?
the paper
Why should the start line be drawn in pencil and not in ink?
Pencil does not dissolve in the solvent, but ink would dissolve and move up the paper.
Sample questions
Written for this site in the style of AQA exam questions. They are not taken from real past papers.
Question 1Easy4 marks
(a) In paper chromatography, what is the mobile phase? Tick (✓) one box.[1]
The paper
The solvent
The start line
The spots of dye
(b) Why is the start line on a chromatogram drawn in pencil and not in ink?[1]
(c) A pure substance is tested using three different solvents. How many spots would you expect on each chromatogram?[1]
(d) Complete the equation for the Rf value of a substance. Rf = ....................[1]
Show the answer and mark scheme
(a)Answer: The solvent
(b)
pencil does not dissolve in the solvent / pencil will not move up the paper
(c)Answer: 1
1 / one (in each solvent)
(d)Answer: distance moved by substance ÷ distance moved by solvent
(Rf =) distance moved by substance ÷ distance moved by solvent
Question 2Medium5 marks
A student used paper chromatography to find out how many dyes are in a food colouring. The student put a spot of the food colouring on a pencil line 2 cm from the bottom of the paper. The student placed the paper in a beaker containing water to a depth of 1 cm and put a lid on the beaker.
(a) Give one reason why the water must be below the pencil line.[1]
(b) Give one reason why the student put a lid on the beaker.[1]
(c) The water moved 7.5 cm from the pencil line. One dye moved 5.4 cm. Calculate the Rf value of this dye. Give your answer to 2 significant figures.[2]
(d) One dye did not move from the pencil line. Suggest why.[1]
Show the answer and mark scheme
(a)
so that the dyes do not dissolve into the water in the beaker
(b)
to stop the water / solvent evaporating
(c)Answer: 0.72
5.4 ÷ 7.5
0.72
(d)
the dye is insoluble in water / in the solvent
Question 3Hard7 marks
A student wants to find out whether the black ink in a pen is a mixture and whether it contains any of three known dyes, J, K and L.
(a) What is the stationary phase in paper chromatography?[1]
(b) Describe how the student could use paper chromatography to find out whether the black ink is a mixture and whether it contains J, K or L. You should include how the student would use the results.[6]
Show the answer and mark scheme
(a)Answer: the paper
(the) paper
(b)
draw a start line in pencil about 1–2 cm from the bottom of the chromatography paper
put small spots of the black ink and of dyes J, K and L on the start line, spaced apart
stand the paper in a beaker of solvent (e.g. water) with the solvent level below the start line
put a lid on the beaker to stop the solvent evaporating
remove the paper before the solvent reaches the top, mark the solvent front in pencil and let the paper dry
if the black ink separates into more than one spot it is a mixture
a spot from the ink at the same height as the spot of J, K or L shows that the dye is in the ink
measure the distances moved by each spot and by the solvent from the start line and compare Rf values
repeat with a different solvent to confirm the identification / use another solvent if spots stay on the start line
Marked with levels of response: the full level descriptors are in the app.