Paper chromatography — required practical
AQA GCSE Chemistry
Separate and identify the substances in a mixture (e.g. inks or food colourings) and calculate Rf values.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Method
- Draw a start line in pencil near the bottom of the chromatography paper and spot the samples on it.
- Stand the paper in a beaker with solvent below the start line; cover it.
- Let the solvent rise until near the top, then mark the solvent front and let the paper dry.
- Measure the distances moved by each spot and by the solvent.
Variables
- Independent variable: The mixture spotted
- Dependent variable: The distance each substance travels (Rf value)
- Control variables: Solvent, paper type, temperature
Results & analysis
Rf = distance moved by the spot ÷ distance moved by the solvent. Rf is always between 0 and 1 and is characteristic of a substance in a given solvent, so it can identify components.
Risk assessment
- Some solvents are flammable/harmful — keep away from flames; ventilate.
Improving accuracy & reliability
- Draw the start line in pencil (ink would run).
- Keep the start line above the solvent; cover the beaker to stop evaporation.
▶ Practise a required-practical exam question and get it marked (free) →