3. Oxidizing action. According to its formula, nitric acid contains a large percentage of oxygen, and the reaction just mentioned shows that the compound is not a very stable one, easily undergoing decomposition. These properties should make it a good oxidizing agent, and we find that this is the case. Under ordinary circumstances, when acting as an oxidizing agent, it is decomposed according to the equation
2HNO3 = H2O + 2NO + 3O.
The oxygen is taken up by the substance oxidized, and not set free, as is indicated in the equation. Thus, if carbon is oxidized by nitric acid, the oxygen combines with carbon, forming carbon dioxide (CO2):
C + 2O = CO2.
4. Action on metals. We have seen that when an acid acts upon a metal hydrogen is set free. Accordingly, when nitric acid acts upon a metal, such as copper, we should expect the reaction to take place which is expressed in the equation
Cu + 2HNO3 = Cu(NO3)2 + 2H.
This reaction does take place, but the hydrogen set free is immediately oxidized to water by another portion of the nitric acid according to the equation
HNO3 + 3H = 2H2O + NO.
As these two equations are written, two atoms of hydrogen are given off in the first equation, while three are used up in the second. In order that the hydrogen may be equal in the two equations, we must multiply the first by 3 and the second by 2. We shall then have
3Cu + 6HNO3 = 3Cu(NO3)2 + 6H,