Different substances travel through a piece of chromatography paper at different speeds.
Paper chromatography uses this to separate mixtures of soluble substances. It provides clues on the possible identity of the substances in the mixture. These substances are often coloured, such as food colourings, inks, dyes or plant pigments.
Chromatography relies on two phases, each with different properties. In paper chromatography, one phase is the paper (the stationary phase) and the other phase is the solvent (the mobile phase).
The dissolved substances move at different rates because the strength of their attraction to each phase is different. Substances that have a strong attraction to the paper move slowly and only travel a short distance. Substances that have a strong attraction to the solvent move quickly and travel further.
A paper chromatogram can by used to distinguish between pure and impure substances.
It can also help to identify substances by comparing them to known substances. Two substances are likely to be the same if:
The Rf value is a measure of the distance the substance travels, relative to the distance travelled by the solvent in the paper. You can identify an unknown substance by comparing its Rf value to the Rf values of a range of known substances.
Rf values vary from 0 (the substance is not attracted to the solvent) to 1 (the substance is not attracted to the paper).