Required Practical: Chromatography — AQA GCSE Chemistry
This required practical uses paper chromatography to separate and identify the substances in a mixture.
The principle
Paper chromatography separates substances based on how well they dissolve in the solvent and how strongly they are attracted to the paper:
- Stationary phase — the chromatography paper.
- Mobile phase — the solvent that moves up the paper.
Substances more attracted to the solvent travel further; those more attracted to the paper travel less.
Method
- Draw a start line in pencil near the bottom of the paper (pencil doesn't dissolve).
- Place small spots of the mixtures (e.g. food colourings) on the line.
- Stand the paper in a container with a shallow depth of solvent, keeping the start line above the solvent level.
- Let the solvent rise up the paper, carrying the substances with it.
- Remove the paper before the solvent reaches the top; mark the solvent front in pencil and let it dry.
Rf values
Each substance has an Rf value under the same conditions: $$R_f = \frac{\text{distance moved by substance}}{\text{distance moved by solvent}}$$ Rf is always between 0 and 1. A pure substance gives one spot; a mixture gives several.
Exam tips
- Draw the start line in pencil and keep it above the solvent.
- Learn the Rf equation and that Rf is between 0 and 1.
- A pure substance gives a single spot.
- Substances separate by their solubility and attraction to the paper.