Nitric acid attacks all the metals except noble metals but the nature of products of same metal along with corresponding nitrates depends upon the concentration of `HNO_3`. Explain.

Explanation

In conc. `HNO_(3)`, more number of `NO_(3)^(-)` ions is present per unit volume, and these `NO_(3)^(-)` ions acts as an oxidizing agent. It gets reduced to `NO_(2)` due to more production of oxygen. In case of dilute `HNO_(3)`, less number of `NO_(3)^(-)` ions is present. So, it gets reduced to nitric oxide.
`Cu+4HNO_(3) to underset(("conc.")) (Cu(NO_(3))_(2)) +2NO_(2) +2H_(2)O`
`3Cu+8HNO_(3) to underset(("dil."))(3Cu(NO_(3))_(2)) +2NO +4H_(2)O`