• Filtration: an insoluble solid from a liquid (sand from water).
• Crystallisation: a soluble solid from its solution: heat to evaporate some water, cool, filter off the crystals, dry them.
• Simple distillation: a liquid from a solution (pure water from sea water).
• Fractional distillation: liquids with different boiling points. The one with the lowest boiling point reaches the top of the column first.
• Paper chromatography: the soluble substances in a mixture, such as the dyes in an ink.

Spot the mixture on a pencil line, stand the paper in solvent, and let the solvent rise. Each substance moves a different distance. A pure substance gives one spot.

Rf = distance moved by the substance ÷ distance moved by the solvent

1. Solvent front 8.0 cm, spot 3.2 cm
2. Rf = 3.2 ÷ 8.0 = 0.40

Rf is always less than 1. The same substance has the same Rf in the same solvent, so matching Rf values identifies substances.

Draw the start line in pencil: ink would run with the solvent.