A compound is a pure substance formed from two or more elements chemically combined in a fixed ratio, so that it contains two or more types of atoms. Because the elements are chemically bonded rather than merely mixed, a compound has properties quite different from those of its component elements and can only be separated into them by chemical means. For example, water (H₂O) is a compound of hydrogen and oxygen, in contrast to a mixture, whose composition can vary.

化合物是由两种或多种元素以固定比例化学结合而成的纯净物,因而含有两种或多种原子。由于其中的元素是化学成键而非简单混合,化合物的性质与组成元素截然不同,只能用化学方法分解为这些元素。例如,水(H₂O)是氢与氧的化合物;与之相对,混合物的组成则可以变化。