The separation of mixtures produced in chemical reactions is often carried out by passing the mixture through a stationary phase of silica held in a column. Solvents move compounds at different rates through the silica allowing them to be separated into fractions by collecting the emerging solvent. This can be carried out by allowing the solvent to flow under gravity or under a moderate pressure to increase the solvent flow rate ('flash chromatography'). The fractions are usually analysed by TLC in order to identify which components of the mixture they contain.