1) Concentrated nitric acid , upon long standing turns yellow-brown due to the formation of A) NO B) $NO_{2}$ C) $N_{2}O$ D) $N_{2}O_{4}$ Answer: Option BExplanation:Plan NO2 is a brown coloured gas and imparts this colour to concentrated HNO3 due to long-standing $4HNO_{3}\rightarrow 2H_{2}0+2NO_{2}+3O_{2}$