The reddish brown coloured gas formed when nitric oxide is oxidised by air is

Solution :
$$\mathop {NO}\limits_{{\text{Nitric}}\,{\text{Oxide}}\left( {{\text{colourless}}} \right)} + {O_2} \to \mathop {N{O_2}}\limits_{{\text{Nitrogen}}\,{\text{dioxide}}\,\left( {{\text{reddish}}\,{\text{brown}}\,{\text{coloured}}} \right)} $$