Question

Concentrated nitric acid, upon long standing, turns yellow-brown due to the formation of:
a.) NO
b.) $N{{O}_{2}}$
c.) ${{N}_{2}}{{O}_{{}}}$
d.) ${{N}_{2}}{{O}_{2}}$

Answer 1

The nitric acid also known as aqua forts and the spirit of niter. It is a colorless compound in its pure state. It generates a reddish brown fumes which has a suffocation odor. It is a hygroscopic liquid.

Complete step by step answer:
Nitric acid has the chemical formula as $HN{{O}_{3}}$. Nitric acid has a molar mass of 63.012$gmo{{l}^{-1}}$. Two forms of nitric acid are available commercially of which most common is nitric acid which has a concentration of 68% in water. When solution has concentration of more than 86% then it is known as fuming nitric acid.
Nitric acid is the primary reagent which is used for adding a nitro group to an organic molecule i.e. nitration of an organic compound.
Nitric acid when left standing in a glass bottle turns yellow due to the decomposition because it is unstable to heat in the presence of sunlight liberating nitrogen dioxide which dissolves in water and gives a yellow solution. The reaction involved in the decomposition of nitric acid is:
$4HN{{O}_{3}}\to 2{{H}_{2}}O+{{O}_{2}}\uparrow +4N{{O}_{2}}\uparrow$

After some time the yellow color of nitric acid is converted into the red fuming acid. Nitric acid is also a strong oxidizing agent.
So, the correct answer is “Option B”.

Note: Nitric acid is a strong acid at the ambient temperature. The nitric acid when it is diluted fully dissociated into ions, except the extremely acidic conditions. Nitric acid reacts with most of the metal but the reaction depends on the concentration and the nature of the metal. Some metals like gold and platinum do not react with nitric acid.