Question

Explain the following:
Concentrated nitric acid appears yellow when it is left standing in a glass bottle.

Answer 1

Hint: Nitric acid, also known as aqua fortis and spirit of niter, is a colorless compound when in the pure state. It generates reddish brown fumes with a suffocating odor. It is a hygroscopic corrosive liquid.

Complete step by step answer:
-Nitric acid has a chemical formula as $HN{{O}_{3}}$ with molar mass as $63.012g mo{{l}^{-1}}$ .
-There are two forms of nitric acid commercially available, of which most common is nitric acid which has a concentration of 68% in water. When the solution contains more than 86% nitric acid, it is known as fuming nitric acid.
-Depending on the amount of nitrogen dioxide present in fuming nitric acid, it is further classified as red fuming nitric acid which has concentrations above 86% pr white fuming nitric acid which has a concentration above 95%.
-Nitric acid is a primary reagent used for adding a nitro group to an organic molecule, i.e nitration of organic compounds.
-Nitric acid when left standing in a glass bottle turns yellow due to decomposition as it is unstable to heat and in the presence of sunlight liberating nitrogen dioxide which dissolves in water and gives a yellow solution. The reaction of decomposition of nitric acid is given as –
\[4HN{{O}_{3}}\to 2{{H}_{2}}O+{{O}_{2}}\uparrow +4N{{O}_{2}}\uparrow \]

After some time, the yellow nitric acid is converted into red fuming acid.
-Nitric acid is also a strong oxidizing agent.

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