AQA GCSE Combined Science Foundation (8464), Foundation tier · Chemistry › Chemical analysis › Purity, formulations and chromatography
Practise Chromatography. 9 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.
Paper chromatography separates the substances in a mixture and helps identify them using Rf values. It is a required practical, so expect questions on the method, interpreting chromatograms and calculating Rf values.
Key facts
Stationary phase = paper; mobile phase = solvent
Rf = distance moved by substance ÷ distance moved by solvent
Measure from the start line to the centre of the spot
Rf has no units and is between 0 and 1
Pure compound: a single spot in all solvents
Mixture: two or more spots
Same substance: same Rf value in the same solvent
Start line in pencil; solvent level below the start line; lid on
Notes
How paper chromatography works
Chromatography separates the substances in a mixture and gives information to help identify them.
It has a stationary phase, which does not move (the paper), and a mobile phase, which moves (the solvent).
As the solvent moves up the paper it carries the substances with it. Different substances move different distances, so a mixture separates into several spots.
A substance that does not dissolve in the solvent stays on the start line.
Rf values
diagram
Rf = distance moved by substance ÷ distance moved by solvent.
Measure both distances from the start line: to the centre of the spot, and to the solvent front.
Measure both distances from the start line: to the centre of the spot, and to the solvent front.
Rf has no units and is always between 0 and 1, because a spot cannot move further than the solvent.
Different compounds have different Rf values. A compound's Rf value changes with the solvent, so only compare Rf values measured with the same solvent.
Identifying substances and checking purity
Run the unknown alongside known reference substances on the same paper. A spot at the same height (same Rf) as a reference suggests the same substance.
A pure compound gives a single spot in all solvents. A mixture gives two or more spots.
Required practical: method
diagram
Draw a start line in pencil near the bottom of the paper. Pencil does not dissolve in the solvent; ink would run.
Put small spots of each sample on the line, spaced apart.
Stand the paper in a container of solvent with the solvent level below the start line, so the spots do not dissolve into the solvent. Put a lid on to stop the solvent evaporating.
Solvent level below the pencil line, lid on, spots small and spaced apart.
Take the paper out before the solvent reaches the top, mark the solvent front in pencil straight away, then let it dry.
How to answer each type of question
Interpret a chromatogram
1 to 3 marksGrade 4
Count the spots for each sample: more than one spot means a mixture.
Compare heights with the reference substances: the same height means probably the same substance.
Name the matching substances, and say if any spot matches none of them.
Example. A food colouring, F, was tested on the same chromatogram as four known dyes, A, B, C and D. F produced three spots. Two of its spots were at the same heights as the spots of A and C. The third spot did not match any of the known dyes.
(a) Is F a pure substance? Give a reason. (b) What can you conclude about the dyes in F?
Show the model answerHide the model answer
(a) No, because it produced more than one spot / three spots, so it is a mixture (1) (b) F contains dyes A and C (1). F also contains a dye that is not A, B, C or D (1).
Calculate an Rf value
2 marksGrade 5
Measure (or read) the distance from the start line to the centre of the spot.
Measure (or read) the distance from the start line to the solvent front.
Divide: spot distance ÷ solvent distance.
Round to the significant figures asked for. No units.
Example. On a chromatogram, the solvent front moved 8.4 cm from the start line. The centre of the spot of dye X moved 2.9 cm. Calculate the Rf value of dye X. Give your answer to 2 significant figures.
Show the model answerHide the model answer
Rf = 2.9 ÷ 8.4 (1) = 0.345… = 0.35 (1)
Don’t lose marks
Measuring to the top of the spot, or from the bottom of the paper. Measure from the start line to the centre of the spot.
Giving Rf a unit, or an answer greater than 1.
Drawing the start line in ink: the ink dissolves and gives extra spots.
Putting the solvent above the start line: the samples dissolve into the solvent instead of moving up the paper.
Saying one spot proves a substance is pure. A pure compound gives a single spot in all solvents.
Thinking a spot left on the start line is pure. It means that substance is insoluble in that solvent.
More tips
Memory tricks
Rf = spot ÷ solvent: the smaller distance goes on top, so Rf is always less than 1. If you get more than 1, you divided the wrong way round.
Stationary stays still (paper); mobile moves (solvent).
Same height, same paper, same solvent: probably the same substance.
Exam technique
Write the Rf formula first, then substitute: this can earn a mark even if you misread a distance.
Give Rf values to the significant figures asked for, or 2 significant figures if none are stated.
When asked 'why' about a step, give the reason, e.g. 'lid on, to stop the solvent evaporating'.
When comparing samples, refer to the heights or Rf values and name the substances that match.
What each grade needs
What you need to be able to do, from the first marks up to the top grade.
Grade 3
Tell mixtures from pure substances on chromatogramsA mixture gives two or more spots; a pure substance gives a single spot.
Grade 4
Match spots to known reference substancesSpots of the same substance travel the same distance on the same paper with the same solvent.
Grade 4
Describe the paper chromatography methodPencil start line, small spots, solvent below the line, lid on, mark the solvent front.
Grade 5
Calculate an Rf valueRf = distance moved by substance ÷ distance moved by solvent, both measured from the start line.
Required practical:Chromatography (method, variables and exam tips)
Quick recall
Cover the answers and test yourself. The app has these as flashcards that come back just before you'd forget them.
A pure substance is tested using three different solvents. How many spots would you expect on each chromatogram?
1
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