the crystal hydrate of cw-terpin (the cis form of p-menthan-1,8-diol [I], a dihydroxy alcohol of the terpene class). Terpin hydrate occurs as colorless, odorless, bitter-tasting crystals that are poorly soluble in water and ether but readily soluble in alcohol.

Upon heating, terpin hydrate loses its water of crystallization and is converted into cis-terpin (melting point, 105.5°C). Terpin hydrate is obtained by the action of dilute sulfuric acid on α-pinene; it is used in making terpineol and in medicine as an expectorant. In large doses, the compound acts as an emetic. Terpin hydrate is prescribed for internal use, often in combination with other preparations, for, primarily, chronic bronchitis. It is produced in powder and tablet form.