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chemical compounds formed from oxides of elements and water; one of the main classes of inorganic compounds. Hydroxides are often called oxide hydrates, a name that does not correspond to the nature of hydroxides, since they do not contain separate water molecules. “Hydroxides” is the term that has been adopted in modern international nomenclature.
Hydroxides of almost all chemical elements are known. The hydroxides of many metals are bases, and the hydroxides of nonmetals are oxygen acids. The chemical properties of the bases are determined by the presence of the hydroxyl ion, OH"; those of the acids are determined by the hydrogen ion, H+. This is in accord with the particular way in which the formulas for acids and bases are written—for example, Ba(OH)2 and H2SO4. Hydroxides that have both basic and acidic properties are called amphoteric. The nature of a hydroxide depends on the position of the element in the Mendeleev periodic system. In practice the term “hydroxide” is used only for bases and amphoteric hydroxides.