Paper chromatography
GCSE Chemistry (8462) · Required practical 6 — method, variables, the marks examiners report students losing.
Separate and identify the coloured substances in a mixture (e.g. inks or food dyes) and calculate their Rf values.
Apparatus
- Chromatography paper
- Pencil and ruler
- Beaker with a watch-glass lid
- Suitable solvent (water or ethanol)
- Capillary tube / dropper and the samples
Method
- 1Draw a start line about 1 cm from the bottom of the paper in pencil (ink would run).
- 2Spot each sample on the line as small, concentrated dots and let them dry.
- 3Place the paper in a beaker with the solvent below the start line — the solvent must not touch the spots.
- 4Cover the beaker and let the solvent rise up the paper, carrying the dyes different distances.
- 5Remove the paper before the solvent reaches the top; mark the solvent front in pencil and let it dry.
- 6Measure how far each spot and the solvent front travelled, then calculate Rf = distance moved by the spot / distance moved by the solvent.
Results & processing
- Each dye travels a characteristic distance; Rf = spot distance / solvent-front distance (always less than 1, no units).
- The same substance gives the same Rf under the same conditions, which lets you identify it.
Where students lose marks
Drawing the start line in pen.
Fix: Use pencil — pen ink is itself a mixture that would separate and run, ruining the result.
Starting the spots below the solvent level.
Fix: The solvent must start below the pencil line, or the spots dissolve into the solvent instead of travelling up the paper.
Miscalculating Rf.
Fix: Rf = distance travelled by the spot divided by distance travelled by the solvent front, both measured from the start line.
Improve the method
- Use a lid to stop the solvent evaporating during the run.
- Use small, concentrated spots for tighter, clearer separation.
Try it — exam-style
A dye moves 4.5 cm from the start line while the solvent front moves 6.0 cm. Calculate the Rf value.
Explain why the start line is drawn in pencil rather than pen.
Questions are written in the style of past AQA papers — never copied from them.
Drill it properly
Stuck on paper chromatography?
Rf marks are free once the method and the ratio are second nature — I make sure they are, and your first lesson is free.