Extraction describes the removal of a desired substance from a mixture. An everyday example is brewing tea, where water is the solvent that the desired substances, including caffeine and polyphenols, are soluble in.
A number of different extraction approaches are available, but in many cases a solvent is used in which the desired substance is soluble, but the other components of a mixture are not.
A common way of separating immiscible liquids in the lab is by use of a separating funnel, as shown in the picture below. The more dense liquid sits at the bottom of the funnel whilst the less dense liquid sits in a layer on top of this. The funnel is stoppered, shaken and gas is released by using the tap (this is important as a pressure build up from solvent evaporation could cause the funnel to explode). The bottom layer can then be run off, closing the tap as the meniscus between the other solvent comes closer. More solvent is added to the funnel for further extractions, and the process is repeated until most of the desired substance has been extracted into the new solvent layer.
Compiled by Lucy Bird.