The wet paper: solvent risen by capillary actionGas jar with solvent below the pencil lineWhite tile and ruler to measure RfPaper chromatography. A spot of a mixture sits on a pencil line just above a thin layer of solvent. The solvent climbs the paper by capillary action, fast at first and then slower, and carries the mixture's substances with it; but the paper (and the water held in its fibres) holds each substance back by its own amount, so the more strongly a substance sticks, the slower it moves. The substances separate into spots, each at its own fraction of the solvent's distance: Rf = distance moved by the substance ÷ distance moved by the solvent front, both measured from the base line, always less than 1. Black sketch-pen ink splits into several dyes in water; green food colour separates best in salt water; leaf pigments need petroleum ether and acetone, because chlorophyll does not dissolve in water; colourless lead and cadmium ions need a locating agent sprayed on to be seen. Keep the solvent below the spot, draw the line in pencil and keep the jar covered. Time runs faster than life, as the readout shows.