CBSE Class 12-science Answered

Copper is oxidized by concentrated nitric acid, HNO₃, to produce Cu²⁺ ions; the nitric acid is reduced to nitrogen dioxide, a poisonous brown gas with an irritating odor:

Cu(s)  +  4HNO₃(aq)  ——>  Cu(NO₃)₂(aq)  +  2NO₂(g)  +  2H₂O(l)

When the copper is first oxidized, the solution is very concentrated, and the Cu²⁺ product is initially coordinated to nitrate ions from the nitric acid, giving the solution first a green, and then a greenish-brownish color.  When the solution is diluted with water, water molecules displace the nitrate ions in the coordinate sites around the copper ions, causing the solution to change to a blue color.

In dilute nitric acid, the reaction produces nitric oxide, NO, instead:

3Cu(s)  +  8HNO₃(aq)  ——>  3Cu(NO₃)₂(aq)  +  2NO(g)  +  4H₂O(l)