AQA GCSE Chemistry · Chemical Analysis
Chromatography required practical
Investigate how paper chromatography can separate coloured substances, and calculate Rf values.
The solvent carries each dye up the paper a different distance. Dyes in a mixture that match a known substance at the same height (same Rf) are the same substance.
Equipment
- Chromatography paper and a pencil
- Beaker with a lid (or watch glass)
- Water (the solvent)
- Known food colourings and an unknown mixture
- Capillary tubes and a ruler
Variables
- Independent you change
- The coloured substances being tested
- Dependent you measure
- Distance each spot moves (and its Rf value)
- Control you keep the same
- Same solvent and paper
- Start line above the solvent
- Same size of spot
- Lid on the beaker
Method
- 1
Draw a pencil start line about 1.5 cm from the bottom of the paper.
- 2
Use a capillary tube to put small spots of each known colour and the unknown on the line.
- 3
Pour water into the beaker to a depth below the start line.
- 4
Stand the paper in the beaker and put the lid on.
- 5
When the solvent is near the top, remove the paper and mark the solvent front in pencil. Let it dry.
- 6
Measure how far each spot and the solvent moved, and calculate Rf = distance moved by spot ÷ distance moved by solvent.
Worked example
Solvent front moved 8.0 cm from the start line.
| Spot | Distance moved (cm) | Rf |
|---|---|---|
| Red (known) | 2.4 | 0.30 |
| Blue (known) | 5.6 | 0.70 |
| Yellow (known) | 6.8 | 0.85 |
| Unknown X, spot 1 | 2.4 | 0.30 |
| Unknown X, spot 2 | 5.6 | 0.70 |
Working
Rf = distance moved by substance ÷ distance moved by solvent
Blue: 5.6 ÷ 8.0 = 0.70
Conclusion
Unknown X separates into two spots with the same Rf values as red and blue, so X is a mixture of the red and blue dyes. It contains no yellow. A pure substance would give only one spot.
Risks
- Some inks or solvents are flammable or harmfulUse water as the solvent where possible and keep flames away.
- Food colourings stainWear eye protection and avoid getting dye on skin.
Common mistakes
- Drawing the start line in pen. Ink would dissolve and run; use pencil.
- Putting the solvent above the start line. The spots would dissolve into the solvent in the beaker.
- Measuring to the bottom of the spot. Measure from the start line to the centre of each spot.
Better results
- Keep spots small and concentrated so they don’t spread and overlap.
- Use a lid so the solvent doesn’t evaporate from the paper.
- Try a different solvent if spots don’t separate well, and compare Rf values in the same solvent.
Launching November 2026
7 exam questions on this practical.
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