Purification in a chemical context is the physical separation of a chemical substance of interest from foreign or contaminating substances. The following list of chemical purification methods should not be considered exhaustive.
Filtration is a mechanical method to separate solids from liquids or gases by passing the feed stream through a porous sheet such as a cloth or membrane, which retains the solids and allows the liquid to pass through.
Centrifugation separates solids from liquids, or separates two immiscible liquids, on the basis of density.
Evaporation is used to remove volatile liquids from non-volatile solutes which cannot be done through filtration due to the small size of the substances.
Extraction removes an impurity, or recovers a desired product, by dissolving it in a solvent in which other components of the feed material are insoluble.
Crystallization separates a product from a liquid feedstream, often in extremely pure form, by cooling the feedstream or adding precipitants which lower the solubility of the desired product so that it forms crystals. The pure solid crystals are then separated from the remaining liquor by filtration or centrifugation. In analytical and synthetic chemistry work, purchased reagents of doubtful purity may be recrystallized, e.g. dissolved in a very pure solvent, then crystallized and the crystals recovered, in order to improve and/or verify their purity.
Adsorption removes a soluble impurity from a feedstream by trapping it on the surface of a solid material such as activated carbon which forms strong noncovalent chemical bonds with the impurity. Chromatography employs adsorption and desorption on a packed bed of a solid to purify multiple components of a single feedstream.
Smelting is used to produce metals from raw ore, and involves adding chemicals to the ore and heating it up to the melting point of the metal.
Refining is used primarily in the petroleum industry, whereby crude oil is heated and separated into stages according to the condensation points of the various elements.
Distillation, widely used in petroleum refining and in purification of ethanol separates volatile liquids on the basis of their boiling point.