Concentrated nitric acid, upon long standing, turns yellow-brown due to the formation of :
On standing
HNO3 H2O + NO2 + 1/2 O2
colour of NO2 yellow - brown
Concentrated nitric acid (HNO₃) decomposes slowly over time when exposed to light or heat. The decomposition reaction is:
Nitrogen dioxide (NO₂) is a reddish-brown gas. When it dissolves in the concentrated nitric acid, it imparts a yellow-brown color to the solution. The other gases listed (NO, N₂O, N₂O₄) are colorless. Dinitrogen tetroxide (N₂O₄) is in equilibrium with NO₂ (), but it is the presence of the colored NO₂ molecule that causes the yellow-brown appearance.
Therefore, the correct option is NO₂.
Decomposition of Nitric Acid: Concentrated nitric acid is unstable and decomposes, especially when warm or in light. The primary product responsible for the color is nitrogen dioxide (NO₂).
Nitrogen Dioxide (NO₂): A toxic gas with a characteristic sharp, biting odor and a reddish-brown color. It is a major air pollutant.
Equilibrium: Nitrogen dioxide exists in equilibrium with its dimer, dinitrogen tetroxide (N₂O₄), which is colorless. The brown color is due to the NO₂ molecules: