(a) Nitric oxide turns brown in air. Why ?
(b) Copper dissolves in `HNO_3` but not in `HCl`. Why ?

Explanation

(a) Nitric oxide directly reacts with oxygen of air and forms nitrogen dioxide, `NO_2` which is brown in colour.
`underset (Colourl ess) (2 NO) + underset ((air)) (O_2) rarr underset (Brown coloured gas) (2 NO_2)`.
(b) `HNO_3` behaves as an oxidising agent and oxidises `Cu` to `Cu^(2 +)`, which dissolves in `HNO_3` as copper nitrate. On the other hand, `HCl` does not behave as an oxidising agent and hence copper does not dissolve in `HCl`.