CHEMICAL ACTION is said to occur whenever the prop erties of a substance are so completely altered that we are entitled to regard the product as a new and distinct substance. In the burn ing of coal, for example, a combustible, black solid is chemically changed by union with the oxygen of the air, to produce ash, water vapour and gases, all of which differ from the original coal in all their important properties. The chemist pictures the prop erties of every substance as being determined by the chemical make-up of its molecules, i.e., by the nature of the atoms which are contained in each molecule of the given substance, and the manner of their arrangement. Thus water is different from carbon disulphide because the molecules of water contain atoms which are different from those contained in molecules of carbon disulphide : on the other hand the molecules of grape sugar (glucose) and fruit sugar (fructose) contain identical atoms in identical proportions but the properties of these substances are nevertheless different because the atoms are differently arranged in the two kinds of molecules.
The complete change of properties which is associated with chemical action is therefore to be considered as being accom plished by the recombination or rearrangement of atoms to pro duce new sorts of molecules. The science of chemistry (q.v.) is mainly concerned with the study of chemical change. The details thus far accumulated are commonly presented in chemical text-books under headings that correspond with the various indi vidual elements. The general results and such principles as are generally applicable to chemical change, form the subject-matter of physical chemistry (see CHEMISTRY : Physical), and of these the present article deals with a few which are related most inti mately to the factors that determine how quickly a given chemical change may be accomplished.