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Stability
Pure nitric acid is not very stable. Even at ordinary temperature, in presence of sunlight it undergoes slight decomposition. As the temperature increases, the rate of decomposition also increases. On strong heating it decomposes completely to give nitrogen dioxide, water and oxygen.
The nitrogen dioxide, which is reddish brown in colour, may dissolve in the undecomposed acid to give it an yellowish brown colour.
The above reaction also shows that nitric oxide contains oxygen.Activity
To show that Nitric Acid contains Oxygen
The apparatus is set up as shown in figure 6.15.
The combustion tube is heated strongly and concentrated nitric acid is allowed to flow into it. Due to the heat, it decomposes, liberating the corresponding gases. Reddish yellow fumes of nitrogen dioxide, water vapour and a colourless gas are obtained. Nitrogen dioxide and water vapour dissolve in the water while the colourless gas gets collected in the gas jar. This gas rekindles a glowing splint, which proves it to be oxygen.
Acidic nature
Nitric acid is a strong monobasic acid. It ionizes in water readily as follows:
a) Reactions with Basic Oxides
b) Reaction with Bases (Hydroxides)
c) Reaction with Carbonates and Hydrogen Carbonates
d) Reaction with Metals
Nitric acid usually does not behave as an acid, with metals to form the corresponding salt and liberate hydrogen.
However, magnesium and manganese are the only two metals, which react with cold and very dilute (1%) nitric acid to evolve hydrogen.
