A dye moves 4.5 cm from the start line while the solvent front moves 6.0 cm. Calculate the Rf value.
[2 marks]
Total for this question: 2
Chemistry required practicals
Paper chromatography
AQA 4.8 · RP6
GCSE Chemistry (8462) · Required practical 6 — method, variables, the marks examiners report students losing.
Board and spec code confirmed against AQA 8462 · registry checked 2026-07-17How this checking works
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
Easy
ORIGINAL
Easy
ORIGINAL
Explain why the start line is drawn in pencil rather than pen.
[1 mark]
Total for this question: 1
Questions are written in the style of past AQA papers — never copied from them.
Drill it properly
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