Sometimes more than one substance is dissolved in a solution. Chromatography is a technique that is used to separate those substances. For example, if you take a filter paper and put a dot with a marker pen in the centre of it and then add a drop of solvent on the same point as the one you drew the dot on, you will separate the ink into its different colours and the result will look similar to this:
Chromatography is used by scientists to analyse samples of water in order to find out how much of certain chemicals are in the water. DNA fingerprinting, which was invented by Professor Sir Alex Jeffreys in 1984 at the University of Leicester, is based on chromatography and is used to identify criminals by their DNA samples. Samples of blood or skin can also be compared by the police using chromatography.
The sample liquids to be tested are dropped onto special paper. The bottom of the paper is dipped in water or another suitable solvent. The different-sized particles that make up the samples travel through the paper at different speeds, so they separate at different distances. The chromatogram below tested four ink colours in order to separate them into the colours that were mixed to make them up. Study it carefully, because you will be asked questions about it.