Question

Conc. Nitric acid prepared in laboratory is yellow in colour why? How is this colour removed?

Answer 1

Conc. Nitric acid prepared in the laboratory is yellow in colour due to the dissolution of reddish brown coloured nitrogen dioxide gas in acid. This gas is produced due to the thermal dissociation of a portion of nitric acid.
`4HNO_3 → 2H_2O + 4NO_2 + O_2`
The yellow colour of the acid is removed:
If dry air or CO2 is bubbled through the yellow acid, the acid turns colourless because it drives out NO2 from warm acid which is further oxidized to nitric acid.
By addition of excess of water, nitrogen dioxide gas dissolves in water and thus the yellow colour of the acid is removed.