Every action which causes a change in the internal structure of the molecules is a chemical action. These changes may be of various sorts: either there takes place simply a change of position among the atoms of the molecule and there is then formed a body which is isomeric or metameric with the original substance; or the molecule is broken up into several molecules, and the process is then one of dissociation, or of simple decomposition; or, further, several molecules unite together to form a single molecule, in which case the process is spoken of as one of condensation or addition; or, finally, molecules act upon each other with interchange of atoms and formation of new molecules, which is the phenomenon of mutual decomposition, the most common sort of chemical action.
From the doctrine of the conservation of matter, it follows that the mass of the bodies which take part in any chemical reaction does not change, and is fully accounted for in the products of the reaction: this principle forms the foundation for quantitative chemical investigations into the composition of substances.
From the principle of the conservation of energy, it follows, in like manner, that no energy is lost or created, and that, consequently, the whole of the energy originally present in the bodies, which act upon each other, appears again in the products of the chemical reaction, although often in a different manner than before. This principle forms the foundation of all quantitative thermo-chemical investigations.
The energy of a molecule is always the same at the same temperature; any increase or diminution of the energy of the molecules, without change of their internal structure, manifests itself as an elevation or a lowering of the temperature of the substance in question.
By chemical action, the structure of the molecule is changed, the atoms become grouped together in a different way, new relations manifest themselves among them, and the energy of the new molecule becomes different from that of the original one. The simplest case is that where the only change is in the grouping and the motions of the atoms of the same molecule, that is where an isomeric compound is formed; in such a case the atoms of the molecule pass from one condition of equilibrium to another, and, according as the new condition of equilibrium answers to a greater or less stability than the original, there occurs either an evolution or an absorption of energy, and the temperature of a corresponding body changes; in the first case it rises, in the second case it falls.
The chemical action, the passage from one isomeric condition to the other, is therefore accompanied by an evolution, or by a disappearance of heat, according as the attractions of the atoms are more fully or less fully satisfied in the body in its new condition.
The case is similar in other chemical actions; if the molecules of the substances produced by the reaction contain at the same temperature a smaller total energy than the molecules of the bodies acting