Paper chromatography of food dyes
Run a valid paper chromatogram and compare separated food dyes with known samples under the same conditions.
On this page
Food Chemistry · about 25–35 min
What you need to understand
Let a solvent separate the components, then compare the spots under the same conditions.
Definitions
- paper chromatography
separating dissolved components because they travel different distances with a solvent through paper
- baseline
- the pencil line where sample spots start
- solvent front
- the furthest position reached by the solvent on the paper
- standard
- a known sample run beside an unknown for comparison
Separate first, then compare
A filter cannot separate soluble dyes in a drink: they pass through ordinary filter paper with the liquid. In paper chromatography, a suitable solvent rises through the paper. Components interact differently with the solvent and paper, so they can move different distances and form separated spots.
Draw a pencil baseline and use small, concentrated sample spots. Keep the baseline above the solvent in the container; otherwise the samples can wash straight into that solvent. Pencil does not add soluble ink that could create extra moving spots. Let a spot dry before adding another small drop.
Remove the paper before the solvent reaches the top. Mark the solvent front immediately, before the paper dries. Measure distances from the baseline to the centre of each spot and to the solvent front.
Compare the unknown with standards on the same paper using the same solvent. Matching colours and positions support a match; they do not uniquely prove identity. A change of solvent can change how far each component travels.
Pause and say it: Pencil baseline above the solvent; small spots; mark the front; compare under the same conditions.
Common mistake
Tempting wrong idea: One visible spot proves the sample is pure.
Why it fails: Components may travel together, fail to dissolve, or be invisible in this test. State what the result shows under the conditions used, rather than claiming every possible component has been detected.
Practical work
Practical: Identify dyes in a coloured food mixture
Which known food dyes are present in an unknown soluble mixture?
Safety: Use only approved food dyes and solvent, wear eye protection, keep the beaker covered while it runs and do not taste any sample.
- Change
- food-dye sample placed on the baseline
- Measure
- number, colour and distance of separated spots
- Keep the same
same solvent and paper, same pencil baseline, same spot size, same solvent-front distance
View the apparatus, method and sample results
Apparatus
chromatography paper, pencil and ruler, capillary tubes or clean spotting sticks, covered beaker, safe food-dye standards, unknown food-dye mixture, water.
Method
Draw a pencil baseline near the bottom of the paper and label positions for each standard and the unknown.
Use a clean capillary for each sample to place small, concentrated spots on the baseline. Let each spot dry before adding another small drop.
Stand the paper in a shallow layer of solvent with every spot above the solvent level.
Cover the beaker and let the solvent rise; remove the paper before it reaches the top and mark the solvent front immediately.
Compare the unknown’s spot colours and positions with standards run on the same paper.
| Sample | Spot distance / cm | Solvent distance / cm | Observation |
|---|---|---|---|
| red standard | 3.0 | 6.0 | one red spot |
| blue standard | 4.5 | 6.0 | one blue spot |
| mixture | 3.0 and 4.5 | 6.0 | red and blue spots |
Limitation: Similar spot positions alone may not uniquely identify an unknown substance.
Improvement: Run known standards beside the sample under identical conditions and repeat with another solvent if needed.
Work through the evidence
Modelled example 1
Read the dye comparison
Problem
In the illustrative chromatogram, the mixture gives a red spot 3.0 cm from the baseline and a blue spot 4.5 cm from it. The red and blue standards reach those same positions on the same paper. What conclusion is supported, and what is its limit?
Study the worked solution
Compare like with like
Method
Compare both colour and distance for each mixture spot.
Reason
The standards ran with the mixture under the same conditions.
Working
Red matches red at 3.0 cm; blue matches blue at 4.5 cm.
State a supported conclusion
Method
The mixture is consistent with containing both standard dyes.
Reason
Different substances can sometimes produce matching spots, so this comparison is not unique proof of identity.
Working
Two separated spots show at least two separated components in this test.
Examiner practice 2
Separate dyes by paper chromatography
Practice question
Describe how to use paper chromatography to separate the soluble dyes in a drink. Include two method details that prevent an invalid result. [4 marks]
Write a four-mark method
View solution step by step
Prepare the paper
2 marksMethod
Draw a pencil baseline and place a small sample spot on it.
Reason
Pencil does not dissolve to add extra spots, and a small spot gives clearer separation.
Working
Use pencil; let the spot dry if another small drop is added.
Run and record
2 marksMethod
Stand the paper with the baseline above the solvent, allow the solvent to rise, then mark its front.
Reason
A submerged sample washes into the solvent, while an unmarked solvent front may disappear during drying.
Working
Remove before the solvent reaches the top and observe the separated spots.
Self-mark with the mark scheme
Compare your response with each mark point. Select a point only when your response contains that evidence.
Self-mark pencil, sample placement, solvent position and observation.
Challenge 3
Recover water and an insoluble herb
Problem
A food mixture contains an insoluble herb, water and a dissolved colouring. The herb and water must be collected, and the colouring only needs to be identified. Choose a sensible sequence of methods and explain each choice.
Work backwards from the required products
Hints
Hint 1: remove the insoluble solid
Begin with the property that distinguishes the herb.
Hint 2: treat the filtrate by purpose
Collect the solvent with distillation; identify soluble colours with chromatography on a separate sample.
View solution step by step
Collect the herb
Method
Filter the mixture.Reason
The herb is insoluble and remains as residue.Working
Wash and dry the residue if a clean dry herb sample is required.
Collect water and identify colouring
Method
Distil part of the filtrate to collect water and run paper chromatography on a small separate sample.
Reason
Distillation recovers the solvent; chromatography separates or identifies dissolved colour components.
Working
Do not use filtration again for the dissolved colouring.
Practise and continue
Practise this
A learner uses a pen baseline, leaves the sample under the solvent level, and forgets to mark the solvent front. Explain the effect of each mistake.
Check your answer
Pen ink may dissolve and create extra spots. A submerged sample may wash into the solvent in the container. Without the front marked before drying, its distance may be unavailable for comparison.
Think like a scientist
Why must a chromatography baseline be drawn in pencil above the solvent?
Check the reasoning
Pencil graphite does not dissolve and add moving spots, while keeping the samples above the solvent lets the rising solvent carry them through the paper.
Remember: Mark the solvent front before it evaporates if distances will be compared.
One-minute check
- Why can chromatography separate dissolved dyes that filtration cannot?
- Why must the pencil baseline and its sample spots be above the solvent?
What makes comparison with a standard fair, and why is a matching spot not unique proof of identity?
Syllabus and review details
- SEC G1 Science 2027 · 2027
Content structure and syllabus content, PDF pages 8–17
Last reviewed: