A chromatogram separates a mixture of dissolved substances into spots. To interpret it, count the spots, measure each from the baseline, and compare with reference substances run on the same paper.
This lesson follows choosing filtration or evaporation because chromatography handles mixtures where everything is dissolved and filtering cannot help. It sits inside separation and chemical analysis.
What does a chromatogram show?
A small spot of the sample is placed on a pencil baseline near the bottom of the paper. The bottom edge of the paper dips into a solvent, which rises up by capillary action and carries the substances with it. Substances that dissolve well in the solvent and cling less to the paper travel further.
Two facts follow. A pure substance gives one spot in a suitable solvent. A mixture gives more than one spot, at least when its parts travel at different speeds.
How do you read and use Rf values?
- Mark the solvent front in pencil as soon as the paper is removed, before it dries.
- Measure from the baseline, not from the bottom of the paper, to the centre of each spot and to the solvent front.
- Calculate Rf = distance moved by spot ÷ distance moved by solvent front.
- Compare each Rf with the reference substances run under the same conditions.
- State the conclusion using careful wording: “consistent with” is safer than “is”.
Worked example
Invented data. The solvent front moved 8.0 cm from the baseline. Three reference dyes were run. Dye A moved 2.0 cm, dye B moved 5.6 cm and dye C moved 6.4 cm. An unknown ink showed two spots, at 2.0 cm and at 5.6 cm. What does the ink contain?
Step 1, Rf of each reference: A = 2.0 ÷ 8.0 = 0.25. B = 5.6 ÷ 8.0 = 0.70. C = 6.4 ÷ 8.0 = 0.80.
Step 2, Rf of the ink spots: 2.0 ÷ 8.0 = 0.25 and 5.6 ÷ 8.0 = 0.70.
Step 3, match: the ink spots match A and B. There is no spot at 0.80, so there is no evidence for C.
Step 4, conclusion: the ink is a mixture containing at least two substances, consistent with dyes A and B. It shows no evidence of dye C.
The mistake to watch for
A common slip is measuring from the bottom edge of the paper instead of the baseline.
Mistaken answer: the baseline is 1.5 cm above the paper edge. A spot 3.5 cm above the edge and a solvent front 9.5 cm above the edge give Rf = 3.5 ÷ 9.5 = 0.37.
The student measured both distances from the paper edge.
The correction is to subtract the baseline height. The spot moved 3.5 − 1.5 = 2.0 cm and the front moved 9.5 − 1.5 = 8.0 cm. Then Rf = 2.0 ÷ 8.0 = 0.25.
Check yourself
Try these, then open each answer.
1. The solvent front moved 10.0 cm and a spot moved 4.5 cm. Calculate its Rf.
Show answer
Rf = 4.5 ÷ 10.0 = 0.45. There are no units.
2. A spot has Rf 0.60 and the solvent front moved 12.0 cm. How far did the spot move?
Show answer
Distance = Rf × front distance = 0.60 × 12.0 = 7.2 cm.
3. A sample shows one spot that matches substance X and a second spot that matches nothing on the reference list. What can you conclude?
Show answer
The sample is a mixture of at least two substances. One is consistent with X, and the other is not one of the listed references.
Where this leads next
Next, use a stated test result to narrow an unknown, where the evidence comes from chemical tests instead of distances. The mixed practice set includes Rf questions at different levels.
If you can calculate Rf but lose marks on the final sentence, our teachers can work on that in online one-to-one Chemistry tuition.