Copper is oxidized by concentrated nitric acid, HN#O_3#, to produce #Cu^(2+)# ions; the nitric acid is reduced to nitrogen dioxide, a poisonous brown gas with an irritating odor:
Cu(s) + 4HN#O_3#(aq) ——> Cu#(N#O3#)_2#(aq) + 2N#O_2#(g) + 2#H_2#O(l)
When the copper is first oxidized, the solution is very concentrated, and the #Cu^(2+)# 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) + 8HN#O_3#(aq) ——> 3Cu(#NO3)_2#(aq) + 2NO(g) + 4H2O(l)
b) the solution changes from colorless to red
c) a cloud of red toxic gas evolves around the beaker
d) copper is eventually completely consumed
