In chemistry, extraction is a method of separating a desired substance from a solution containing one or more other substances. The solution is shaken with a solvent immiscible with it, into which the solute required (but not the others) is largely transferred, according to the distribution law that its concentrations in the two phases are in a constant ratio. The process is repeated if necessary. Continuous extraction is used if the distribution ratio is small. Metal ions are extracted as a chelate by adding a ligand to the solution.