Question

The colourless gas that turns brown in air is

• A. $NO$
• B. $NO_{2}$
• C. $N_{2}O_{4}$
• D. $N_{2}O_{5}$

Answer 1

Answer: (A)

$NO$

Answer 2

Nitric oxide, $NO$, a colourless gas, when comes in contact of air, turns brown due to its oxidation into $NO _{2}$ (which is brown in colour).
$\underset{\text { colourless }}{2 NO }+\underset{\text { (fromair) }}{ O _{2}} \rightarrow \underset{\text { brown }}{2 NO _{2}}$