Chemistry teaches that the thousands of forms of matter upon the earth, almost infinite in variety, can be resolved into eighty substances, unalterable by chemical processes and possessing definite spectra. These, substances are called elements. The metals, iron, gold, silver and others, sulphur, and carbon are familiar example of elements. A mass of an element is made up of a collection of molecules. Each molecule of an element as a rule is composed of two atoms.
Elements combine to form compound substances of various numbers of atoms in the molecule.
Water is an example of a compound substance, or chemical compound. Its molecule contains three atoms, two atoms of hydrogen, and one atom of oxygen. If a quantity of these two elements were mixed, the result would be a mechanical mixture of the molecules of the two. But if heat, or some other adequate cause were made to act, chemical action would follow and the molecules, splitting up, would combine atom with atom. The result would be the production of a quantity of molecules of water.