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Learn5070Paper 2 · TheoryExperimental techniques and chemical analysisChromatography
Experimental techniques and chemical analysis · Subtopic 2 of 3

Chromatography

Paper chromatography separates soluble substances. Spots are placed on a pencil baseline above the solvent level; the solvent moves up the paper, carrying each substance a different distance depending on its solubility and attraction to the paper. A pure substance gives one spot. Substances are identified by comparing with known samples, or by the RfR_f value =distance moved by substancedistance moved by solvent=\frac{\text{distance moved by substance}}{\text{distance moved by solvent}}. Colourless substances need a locating agent.

Cambridge 5070 Paper 1Cambridge 5070 Paper 2Cambridge 0620 Papers 2, 4Cambridge 0620 Papers 1, 3Edexcel 4CH1 Paper 1CEdexcel 4CH1 Paper 2C≈ 30 minRevision 1 · updated 2026-10-30
Section 1 of 14

Learning objectives

  • Describe how to carry out paper chromatography.
  • Interpret chromatograms to identify substances and check purity.
  • Calculate RfR_f values.
Pause and think

A black felt-tip pen line on wet paper spreads into blue, red and yellow bands. What does that tell you about the ink?

Section 2 of 14

Core idea

Start hereHidden colours

A green sweet coating might be one green dye or a blue and a yellow mixed. Chromatography lets you see what's really there.

The big idea

Substances move up the paper at different rates depending on how soluble they are in the solvent and how strongly they stick to the paper. RfR_f values identify them.

1. Setting up

Paper chromatography method

Each step avoids a specific error.

2. RfR_f values

Worked exampleIdentifying amino acids

After spraying with ninhydrin, a spot is 3.0 cm from the baseline; the solvent front is 7.5 cm. Glycine has Rf=0.40R_f=0.40.

Goal
Is the spot glycine?
Strategy
Calculate RfR_f and compare.
  1. Rf=3.07.5R_f=\frac{3.0}{7.5}
    Spot ÷ solvent.
  2. =0.40=0.40
    Matches glycine.
AnswerProbably glycine (confirm with a second solvent).
✓ CheckBetween 0 and 1 ✓.
Edge caseSame RfR_f isn't proof Two different substances can share an RfR_f in one solvent. Running in a second solvent makes identification much more reliable.
Pause and think

Why must the spot be small and concentrated?

Summary

  • Pencil baseline above solvent.
  • One spot = pure.
  • RfR_f = spot ÷ solvent.
Key words
Chromatogram
The developed chromatography paper.
Solvent front
How far the solvent travelled.
Locating agent
Spray that reveals colourless spots.
HistoryColour writing. Russian botanist Mikhail Tsvet invented chromatography in 1903 to separate plant pigments. The name means "colour writing".
Fun factForensic ink. Forensic scientists use chromatography to check whether two parts of a document were written with the same pen.
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