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

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Solution

Conc. Nitric acid prepared in the laboratory is yellow in color due to the dissolution of reddish-brown colored nitrogen dioxide gas in acid. This gas is produced due to the thermal dissociation of a portion of nitric acid.
$4 H N O_{3} ⟶ 2 H_{2} O + 4 N O_{2} + O_{2}$
The yellow color of the acid is removed: If dry air or $C O_{2}$ is bubbled through the yellow acid, the acid turns colorless because it drives out $N O_{2}$ from warm acid which is further oxidized to nitric acid. By the addition of an excess of water, nitrogen dioxide gas dissolves in water and thus the yellow color of the acid is removed.

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