|Samples of copper, aluminum and zinc; addition of nitric acid||Only copper shows a distinct reaction|
|Photo 1:||Experimental setup: Concentrated nitric acid is simultaneously added to copper and aluminum samples. Diluted nitric acid is added to zinc.
Immediately thereafter, zinc and aluminum show no reaction. Copper, on the other hand, reacts immediately and produces brown NO2 according to:|
Cu + 2 HNO3 + 2 H+ > Cu2+ + 2 NO2 + 2 H2O
|Photo 2:||Aluminum does not react; there is a barely perceptible reaction with zinc; and a strong reaction with copper continues.|
Concentrated nitric acid is a very strong oxidator capable of reacting with the noble metals such as copper or silver (not gold, however). Aluminum and zinc do not react with concentrated nitric acid since a thick, hard to dissolve oxidation layer builds (passive oxidation) that protects the metal against further assaults. Zinc, however, reacts with diluted nitric acid (for an analog, see Experiment 18.104.22.168).