3MnO2 = Mn3O4 + 2O.
Classes of reactions. When a chemical change takes place in a substance the substance is said to undergo a reaction. Although a great many different reactions will be met in the study of chemistry, they may all be grouped under the following heads.
1. Addition. This is the simplest kind of chemical action. It consists in the union of two or more substances to produce a new substance. The combination of iron with sulphur is an example:
Fe + S = FeS.
2. Decomposition. This is the reverse of addition, the substance undergoing reaction being parted into its constituents. The decomposition of mercuric oxide is an example: HgO = Hg + O.
3. Substitution. It is sometimes possible for an element in the free state to act upon a compound in such a way that it takes the place of one of the elements of the compound, liberating it in turn. In the study of the element hydrogen it was pointed out that hydrogen is most conveniently prepared by the action of sulphuric or hydrochloric acid upon zinc. When sulphuric acid is used a substance called zinc sulphate, having the composition represented by the formula ZnSO4, is formed together with hydrogen. The equation is
Zn + H2SO4 = ZnSO4 + 2H.
When hydrochloric acid is used zinc chloride and hydrogen are the products of reaction:
Zn + 2HCl = ZnCl2 + 2H.
When iron is used in place of zinc the equation is