hydroxideion

The hydroxide ion, OH−, is a negatively charged polyatomic ion consisting of one oxygen atom covalently bonded to one hydrogen atom. The overall charge is −1, and the negative charge is primarily localized on the oxygen atom. It is the conjugate base of water (H2O) and the conjugate acid of oxide (O2−) in acid–base equilibria.

In aqueous solution, hydroxide ions are produced when bases dissolve and release OH−, or when water self-ionizes to a small extent. They are also released from solids that contain hydroxide groups, such as the minerals brucite (Mg(OH)2) and goethite (FeO(OH)). In solution, OH− imparts basicity and raises pH.

Chemical reactivity of OH− includes neutralizing acids to form water, precipitating metal hydroxides with certain metal ions, and converting dissolved carbon dioxide into carbonate species (CO2 + OH− → HCO3−; HCO3− + OH− → CO3^2−). In organic chemistry, hydroxide acts as a common nucleophile and base for substitution and elimination reactions, especially in aqueous or alcoholic media.

Applications and significance: hydroxide is central to acid–base chemistry, water treatment, and many industrial processes. It is used to adjust pH, in electrochemistry, and in the production of various hydroxide salts. It also plays a role in environmental cycles, such as carbonate buffering in natural waters.

Safety: hydroxide solutions, particularly concentrated ones, are caustic and can cause severe burns on skin or eyes. They react vigorously with acids and oxidizing agents. Proper handling requires protective equipment and storage in compatible containers.