Chromatography relies on two different 'phases':
The different dissolved substances in a mixture are attracted to the two phases to different degrees. This causes them to move at different rates through the paper.
Separation by chromatography produces a chromatogram.
A paper chromatogram can be used to distinguish between pure and impure substances:
A paper chromatogram can also be used to identify substances by comparing them with known substances. Two substances are likely to be the same if:
In this chromatogram, the brown ink is made of a mixture of the red, blue and yellow inks. This is because the spots in the brown ink reach the same heights (and have the same Rf value) as the reference inks.
Rf values can be used to identify unknown chemicals if they can be compared to a range of reference substances. The Rf value is always the same for a particular substance if run in the same solvent system in the same conditions.
The Rf value of a spot is calculated using:
Rf values vary from 0 (the substance is not attracted at all to the mobile phase) to 1 (the substance is not attracted to the stationary phase).
Different compounds have different Rf values in different solvents, which can be used to identify them. The compounds in a mixture may separate into different spots depending on the solvent, but a pure compound produces a single spot in all solvents.