N2O3 + H2O = 2HNO2,
N2O5 + H2O = 2HNO3.
On the other hand, nitrous acid very easily decomposes, yielding water and nitrogen trioxide, and by suitable means nitric acid likewise may be decomposed into water and nitrogen pentoxide:
2HNO2 = H2O + N2O3,
2HNO3 = H2O + N2O5.
In view of the close relation between these oxides and the corresponding acids, they are called anhydrides of the acids, N2O3 being nitrous anhydride and N2O5 nitric anhydride.
DEFINITION: Any oxide which will combine with water to form an acid, or which together with water is formed by the decomposition of an acid, is called an anhydride of that acid.
EXERCISES
1. Perfectly dry ammonia does not affect litmus paper. Explain.
2. Can ammonia be dried by passing the gas through concentrated sulphuric acid? Explain.