Nitrous acid (HNO2). It is an easy matter to obtain sodium nitrite (NaNO2), as the reaction given on the previous page indicates. Instead of merely heating the nitrate, it is better to heat it together with a mild reducing agent, such as lead, when the reaction takes place which is expressed by the equation
NaNO3 + Pb = PbO + NaNO2.
When sodium nitrite is treated with an acid, such as sulphuric acid, it is decomposed and nitrous acid is set free:
NaNO2 + H2SO4 = NaHSO4 + HNO2.
The acid is very unstable, however, and decomposes readily into water and nitrogen trioxide (N2O3):
2HNO2 = H2O + N2O3.
Dilute solutions of the acid, however, can be obtained.
COMPOUNDS OF NITROGEN WITH OXYGEN
Nitrogen combines with oxygen to form five different oxides. The formulas and names of these are as follows:
| N2O | nitrous oxide. |
| NO | nitric oxide. |
| NO2 | nitrogen peroxide. |
| N2O3 | nitrogen trioxide, or nitrous anhydride. |
| N2O5 | nitrogen pentoxide, or nitric anhydride. |