HYDROXIDE, in chemistry, a compound containing oxygen and hydrogen, linked together in the group, OH, the univalent "hydroxyl" radical, of which one or more are linked to another element or radical. This name has generally replaced the term "hydrate," now confined to substances in which molecules of water as such form a constituent part of the molecule. The above defini tion, taken in the most general sense, includes such compounds as boric acid, B(OH)3 and ethyl alcohol, (qq.v.), where the hydroxyl group is combined with a non-metallic and an organic radical respectively, but in practice the term is generally re stricted to the hydroxides of metallic elements. The most stable hydroxides are those of the alkali metals, lithium, sodium, potas sium, rubidium and caesium. These compounds are powerful alkalis (see ALKALI) ; they are solid substances readily soluble in water to highly alkaline solutions. Sodium hydroxide, also known as caustic soda, and potassium hydroxide, or caustic potash, are the commonest members of this series. Calcium, strontium and barium, the metals of the alkaline earths, also give rise to alkaline hydroxides of which calcium hydroxide, or slaked lime, is the commonest. These three hydroxides are soluble in water to ' give alkaline solutions, of which lime water, which is used in medicine as an astringent and desiccative when applied externally, is the best known example.
Other metals, such as magnesium, copper, cadmium, iron, fur nish hydroxides which are only very sparingly soluble in water. The hydroxides of aluminium, and of zinc, Zn (OH) have both basic and acidic properties ; they dissolve in aqueous solutions of either acids or alkalis. Hydroxides and other sub stances with these dual properties are termed amphoteric com pounds.
See H. B. Weiser, The Hydrous Oxides (1926) .