Edexcel IGCSE Chemistry · Spec 1.11-1.13
Chromatography & Rf Values
How paper chromatography separates mixtures, and how to calculate Rf values.
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Chromatography & Rf Values
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Explained
Separating dyes, and measuring how far they went
Paper chromatography separates a mixture of soluble substances by how they behave between two phases. The paper is the stationary phase and the solvent moving up it is the mobile phase.
A substance that dissolves well in the solvent and clings weakly to the paper travels a long way. One that dissolves less well, or clings more strongly, travels a short way. Different substances therefore end up at different heights, which is what separates them.
Setting it up
Draw a baseline in pencil near the bottom of the paper and put small spots of the mixture on it. Stand the paper in a shallow layer of solvent, making sure the solvent level is below the baseline, and put a lid on the container.
Two of those details carry marks of their own.
The baseline is pencil because pencil is insoluble. Ink would dissolve in the solvent and travel up the paper itself, producing extra spots that confuse the chromatogram.
The solvent must start below the baseline. If it covers the spots, they dissolve straight into the solvent and wash away instead of being carried up the paper.
Rf values
The Rf value is the distance moved by the spot divided by the distance moved by the solvent. Both are measured from the baseline, and the spot distance is measured to the centre of the spot.
Rf has no units, because it is one length divided by another. It is always less than one, because the spot cannot travel further than the solvent carrying it, so an answer above one means the fraction is inverted.
Rf is characteristic of a substance in a particular solvent, so comparing your value with published values identifies it. Change the solvent and the Rf changes, which is why the solvent must be stated.
What the mark scheme accepts and rejects
An Edexcel International GCSE Chemistry mark scheme for the pencil question wants two linked points: that ink would dissolve in the solvent while pencil would not, and that the ink would travel or smudge and produce spots that interfere with the results. It allows the shorter phrasing that ink is soluble and pencil is insoluble, and accepts that you would be unable to calculate an Rf value.
The Rf calculation is marked in four parts: the measurement to the spot, the measurement to the solvent front, the fraction written out, and the evaluated value. It allows an error carried forward from incorrect measurements, so a misread ruler does not cost the later marks.
The examiner report on that paper identifies where marks were actually lost. Candidates gave the solvent front measurement wrongly, or did not express their final answer to two significant figures. It also notes that multiplying the fraction by 100 was seen, which is a mistake because Rf is a ratio and not a percentage.
On interpreting a chromatogram, another part credits identifying two samples and then explaining that they both contain the dye that travelled furthest, with the second mark depending on the first. Naming the answer without the reason earns half.
Reading a chromatogram
A pure substance produces one spot. A mixture produces several, one for each component.
A substance that has not moved from the baseline is insoluble in that solvent, so it is not separated at all. If a question shows a spot still on the line, that is the explanation.
To identify substances in a mixture, run known reference substances on the same paper alongside it. Spots at the same height are the same substance, and running them together removes any doubt about the solvent or the conditions being identical.
Spec 1.11-1.13
What you need to know
- Describe how chromatography separates a mixture
- Calculate an Rf value
- Use chromatograms to identify substances
Active recall
Quick check
Answer each question before opening the answer.
How is an Rf value calculated?
Rf = distance moved by the spot ÷ distance moved by the solvent.
Why do different substances travel different distances in chromatography?
They have different solubilities and are attracted to the paper by different amounts.
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