Paper chromatography — how it separates
Draw a pencil baseline near the bottom of the paper, place a small spot of the mixture on it, and stand the paper in a shallow layer of solvent kept below the baseline. The solvent soaks up the paper by capillary action, carrying each dissolved substance with it. A substance that is more soluble in the solvent, or less strongly held by the paper, travels further — so the single spot separates into a pattern of spots, the chromatogram.
Reading a chromatogram — identity and purity
Run the unknown alongside known reference substances on the same paper in the same solvent: a spot at the same position ((Extended) the same ) identifies the unknown as that substance. A pure substance gives one spot; an impure substance (a mixture) gives more than one. Strictly, one spot in a single solvent is not absolute proof — two substances could coincidentally travel the same distance — but "one spot = pure" is the rule 0654 requires.
(Extended) Rf — substance distance over solvent distance
Both are measured from the baseline: to the centre of the spot, and to the solvent front. A substance can never overtake the solvent carrying it, so always lies between 0 and 1 and has no unit — the units cancel. A substance has a characteristic in a given solvent, so a measured value can be matched against reference values.
Drawn from real examiner reports.
Baseline in ink, not pencil
The baseline must be drawn in pencil. Graphite is insoluble in the solvent, so the line stays put as a fixed reference for every distance measurement. An ink line dissolves and travels up the paper with the samples, adding an unwanted coloured trace and destroying the fixed point the whole measurement depends on.
Flagged Jun 2023 P42 Q2b; Nov 2023 P42 Q11b
(Extended) Rf inverted — an answer above 1
Substance on top, solvent underneath: . Dividing the solvent distance by the substance distance inverts the ratio and gives a value greater than 1, which is impossible — the solvent always travels at least as far as anything it carries. Treat any above 1 as a signal to go back and check which distance you put on top.
Flagged Jun 2023 P42 Q2b; Nov 2023 P42 Q11b
(Extended) Writing a unit after an Rf value
is a ratio of two distances measured in the same unit, so the units cancel and the answer is a pure number. Writing "Rf = 0.65 cm" is wrong and costs the accuracy mark. Quote the value alone, normally to 2 decimal places. Contrast a rearranged calculation that returns a distance — that answer does carry cm.
(Extended) Dividing centimetres by millimetres
If the spot distance is recorded in millimetres and the solvent front in centimetres (or the other way round), convert both to the same unit before dividing. Dividing mismatched units puts the answer out by a factor of 10 — producing an "Rf" of 7.0 or 0.07 in place of 0.70. Convert first, then substitute.
Measuring from the solvent front
Both distances start at the baseline — not at the solvent front and not at the bottom edge of the paper. The substance distance runs from the baseline to the centre of the spot; the solvent distance runs from the baseline to the solvent front. Measuring the spot downwards from the solvent front returns the complement of the true distance, not the distance asked for.
Purity from spot colour, not spot count
Purity on a chromatogram is judged by how many separate spots appear, never by how dark, large or intensely coloured they are. One spot means a pure substance; more than one spot means a mixture. A single faint spot is still one spot, and a single strong dark spot is not extra evidence of purity.
Solvent level drawn above the baseline
In the set-up the solvent must sit below the baseline. If the solvent level is drawn or set above it, the sample spot dissolves straight into the solvent in the container instead of being carried up the paper, and no chromatogram forms at all. This is the classic error in drawn apparatus diagrams.
Compare only on the same chromatogram
A spot position (or an ) identifies a substance only if unknown and knowns were run on the same paper in the same solvent. Look for an exact match, not the nearest value — if nothing matches, say the unknown is none of the references rather than picking the closest.
(Extended) Rearranged Rf returns a distance
Given and the solvent distance, multiply: distance travelled by substance distance travelled by solvent. Candidates divide out of habit. This answer is a length, so it does carry a unit — quote it in cm.
(Extended) The three-step Rf routine
Write both distances before dividing
The method mark for the correct ratio is awarded separately from the accuracy mark for the value. Quote both measured distances and the division you are performing, then the answer. Jumping straight to a number throws away the method mark whenever the arithmetic slips.
Cambridge 0654 spec reference: Section C12 "Experimental techniques and chemical analysis", sub-topic C12.3 "Chromatography". Core: (1) describe how paper chromatography separates mixtures of soluble coloured substances using a suitable solvent; (2) interpret simple chromatograms to identify unknown substances by comparison with known substances, and to identify pure/impure substances. Supplement (Extended): (3) state and use the equation for .
| Term | Mark-scheme-precise meaning |
|---|---|
| Paper chromatography | A technique used to separate a mixture of soluble coloured substances, by allowing a solvent to travel up absorbent paper carrying the substances at different rates |
| Baseline (origin line) | The starting line for the sample spot(s), drawn in pencil (insoluble in the solvent) near the bottom of the paper, kept above the solvent level |
| Chromatogram | The pattern of separated spots produced once the solvent has travelled up the paper |
| Retention factor, (Extended) | The ratio , both distances measured from the baseline; has no unit and is always between 0 and 1 |
How paper chromatography is carried out:
Interpreting a chromatogram (Core):
| What you see | Conclusion |
|---|---|
| A spot in the unknown sample lines up at the same position (or has the same ) as a spot from a known reference substance run alongside it | The unknown contains that known substance |
| Only ONE spot appears from a sample | The sample is a pure substance |
| MORE THAN ONE spot appears from a sample | The sample is impure -- a mixture of substances |
A dye spot travels 4.8 cm from the baseline while the solvent front travels 8.0 cm. Calculate the value.
In a paper chromatography experiment, a spot of ink travels 6.0 cm from the baseline while the solvent front travels 8.0 cm from the baseline, in the same time.
Calculate the value of the ink spot. (2 marks)
