Paper chromatography: separate and compare coloured solutes
Explain a suitable paper-chromatography setup and compare unknown spots with standards without treating a matching height as absolute proof of identity.
Core and Extended · Chemistry 0620, 2026–2028. These paper-based method exercises support supervised laboratory learning; they do not certify practical competence.
Before you begin
- Distinguish solute and solvent.
- Compare positions measured from the same baseline.
Work on paper. Try each question before revealing help, and explain why your method works.
Quick readiness check
Where should the starting spots be relative to the solvent surface?
What you will learn
- Describe paper chromatography of soluble coloured mixtures in a suitable solvent.
- Interpret unknown and standard spots, and evidence consistent with pure or impure substances.
Keep the sample above the solvent surface
In a supervised paper-chromatography method, small sample spots are placed on a pencil baseline. The baseline and spots begin above the solvent surface, so the sample is carried up the paper by the advancing solvent rather than dissolved into the reservoir. Pencil avoids adding mobile ink components from a pen line. The solvent must dissolve the substances being investigated suitably.
The solvent rises through the paper; different solutes move different distances because their interactions with solvent and paper differ. The sample's components can separate into distinct coloured spots. Mark the solvent front before it becomes lost as the paper dries, and compare samples run under the same conditions. A colour alone, without position and conditions, is not a secure identification.
Use standards and acknowledge the evidence limit
In the supplied diagram, standards A, B and C give spots 2.0, 5.0 and 6.4 cm above the baseline. The unknown U has two resolved spots at 2.0 and 5.0 cm. It is a mixture under this model, with spots matching A and B and no observed match for C. Those matches support the stated identification using the supplied candidate set and shared conditions.
One spot is consistent with a pure substance under the chosen conditions, but it is not absolute proof: components may overlap or fail to be detected. Multiple resolved spots show more than one detected component in the supplied separation. A spot at the same position as a standard is evidence of a possible match, not a unique identity claim for every substance in chemistry.
Pause and explain
An unknown gives one observed spot. What is the strongest justified conclusion?
Worked example
Standards A, B and C move 2.0, 5.0 and 6.4 cm in the same run. Unknown U has spots at 2.0 and 5.0 cm. Interpret U using this candidate set.
Show the worked solution
- Compare every unknown spot with standards measured from the same baseline.
- U's two positions match A and B, with no observed spot matching C.
- Two resolved components indicate a mixture under the supplied model; matching positions support the A/B assignment without proving unique identities beyond the candidate set.
Answer U is a mixture with detected spots matching A and B under the same conditions; no observed match for C.
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Write your answer and reasoning first. Use a hint only if you are stuck. The model solution is for self-checking; your written work is not automatically marked.
Explain why the baseline is drawn in pencil above the solvent and why a suitable solvent is needed.
Give me a hint
Consider added ink and where the sample begins to dissolve.
Compare my reasoning
- Pencil avoids soluble pen-ink components interfering with the observed spots.
- The spots start above the reservoir so the rising solvent carries them through the paper.
- The solvent must dissolve the investigated solutes suitably for separation.
Pencil avoids mobile ink interference; baseline above the solvent prevents washing the sample into the reservoir; suitable solvent dissolves the solutes.
Look for these in your work
- All three method choices have a reason.
- The initial sample position is correctly described.
Use the supplied chromatogram to state U's detected components from A/B/C and whether U appears pure.
Give me a hint
Compare resolved spot positions under the shared conditions.
Compare my reasoning
- U has two resolved spots at 2.0 and 5.0 cm.
- They match standards A and B, with no observed C-position spot.
- U therefore appears to be a mixture of the matched candidates under the supplied model.
U is a mixture with observed matches to A and B, not C, in the supplied candidate set.
Look for these in your work
- Each observed spot is compared.
- The result is limited to the supplied conditions and candidates.
A second solvent produces just one spot from a sample that showed two resolved spots in the first solvent. Does that establish purification? Explain.
Give me a hint
Ask whether changed conditions can change resolution without removing a component.
Compare my reasoning
- Different solvent conditions can change how far components travel and whether they overlap.
- One spot in the second run does not demonstrate that a component was removed.
- The earlier two resolved spots remain evidence of a mixture; further suitable comparison is needed to investigate purity.
No: overlapping or undetected components in the second solvent can give one observed spot without purification.
Look for these in your work
- Resolution and purification are separated.
- One observed spot is not overclaimed as proof.
Common mistakes
- Submerging the sample baseline in the solvent reservoir.
- Drawing the baseline in soluble pen ink.
- Calling one observed spot absolute proof of purity.
- Comparing positions from different conditions as if always equivalent.
What can you explain now?
What do two resolved spots from one sample indicate here?
Compare with the explanation
At least two detected components in the supplied separation.
One starting sample can contain multiple soluble substances that move differently.
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Make it stick
Draw a starting setup with a pencil baseline above solvent, then explain unknown/standard matching and the limits of a single spot.
If you needed the solution, close it and solve the problem again from a blank page. A correct answer is stronger when you can explain the reason for each step.