How can chromatography separate mixtures of colourless substances?

Chromatography separates mixtures of colourless substances by distinguishing their different rates of movement through a stationary phase.

Chromatography is a technique used to separate mixtures into their individual components. Even if the substances are colourless, they can still be separated using this method. The principle behind chromatography is that different substances in a mixture have different affinities for a stationary phase and a mobile phase. The stationary phase is a solid, or a liquid supported on a solid, and the mobile phase is a liquid or a gas.

In a typical chromatography experiment, the mixture is dissolved in a fluid, the mobile phase, which carries it through a structure holding another substance, the stationary phase. The various components of the mixture travel at different speeds, causing them to separate. The separation is based on differential partitioning between the mobile and stationary phases. Subtle differences in a compound's partition coefficient result in differential retention on the stationary phase and thus affect the separation.

Paper chromatography is a common method of separating various components of a mixture. In this method, the stationary phase is a piece of chromatography paper and the mobile phase is a solvent. The mixture is applied near the bottom of the paper and the solvent moves up the paper by capillary action. Different components of the mixture will move at different rates up the paper, depending on how strongly they interact with the paper (the stationary phase) and how soluble they are in the solvent (the mobile phase).

Even if the substances are colourless, they can be detected after the chromatography process. For example, by spraying the chromatogram with a locating agent which reacts with the substance to produce a coloured spot, or by using a UV light, as many substances absorb UV light and appear as dark spots on the chromatogram. This way, chromatography can effectively separate and help identify the components of a mixture, even if they are colourless.