Question

Why Nitric acid turns brown on standing?

Answer 1

• When nitric acid (colorless) is left for long standing, it color changes due to decomposition reaction.
• The reaction in which one reactant yields two or more products, it is called a decomposition reaction.
• Due to decomposition, concentrated nitric acid (${\mathrm{HNO}}_3$) gets broken down to nitrogen dioxide, oxygen and water.
• Due to the formation of nitrogen dioxide, it turns yellow-brown.
• The chemical reaction involved in the reaction is as follows:

$4{\mathrm{HNO}}_3\to 2{\mathrm{H}}_2\mathrm{O}+4{\mathrm{NO}}_2+{\mathrm{O}}_2$