Illustrate how copper metal can give different products on reaction with `HNO_3`.

Explanation

On heating with dil. `HNO_3`, copper gives copper nitrate and nitric oxide.
`3Cu + 8 HNO_3 ("dil.") overset("Heat")to 3Cu(NO_3)_2 + 4H_2 O + underset("Nitric oxide")(2NO)`
With conc. `HNO_3`, instead of NO, `NO_2` is evolved.
`Cu + 4HNO_3 ("conc.") overset("Heat")to Cu(NO_3)_2 + 2H_2 O + underset("Nitrogen dioxide")(2NO_2)`