Fe + H2SO4 = FeSO4 + 2H.
These reactions are quite similar, as is apparent from an examination of the equations. In each case 1 atom of the metal replaces 2 atoms of hydrogen in the acid, and the hydrogen escapes as a gas. When an element in the free state, such as the zinc in the equations just given, takes the place of some one element in a compound, setting it free from chemical combination, the act is called substitution.
Other reactions illustrating substitution are the action of sodium on water,
Na + H2O = NaOH + H;
and the action of heated iron upon water,
3Fe + 4H2O = Fe3O4 + 8H.
4. Double decomposition. When barium dioxide (BaO2) is treated with sulphuric acid two compounds are formed, namely, hydrogen dioxide (H2O2) and barium sulphate (BaSO4). The equation is
BaO2 + H2SO4 = BaSO4 + H2O2.
In this reaction it will be seen that the two elements barium and hydrogen simply exchange places. Such a reaction is called a double decomposition. We shall meet with many examples of this kind of chemical reactions.