Explain the principle of paper chromatography.

The analysis of unknown substances by the flow of solvent on a filter paper is known as paper chromatography. This technique proceeds by a mechanism which is partly partition (distribution) and partly adsorption. The constituents of a mixture are distributed between the water held in the filter paper (water thus acts as a stationary phase) and an organic solvent (mobile phase).
In this method, a drop of the test solution is applied as a small spot near one edge of the filter paper and spot is dried. When the end of the paper strip is dipped into a developing solvent, the solvent rises up the paper by capillary action and flows over the spot. The paper selectively retains different components according to their differing partition in the two phases. The paper strip so developed is known as a chromatogram. The spots of the separated coloured compounds are visible at different heights from the position of the initial spot on the chromatogram. The spots of the separated colourless compounds may be made visible either by ultraviolet light or by the use of a suitable spray reagent.