Question

nitric acid does not produce hydrogen gas when reacts
with metal .
why?

Answer 1

Nitric acid(HNO3) is a strong oxidising agent.
The Hydrogen gas produced during its reaction with metal gets oxidised to H2O, hence no hydrogen gas is produced.
Instead it evolves nitric oxides such as  NO2,NO etc.

For Eg:
Cu(s) + 4HNO3(aq)--->Cu(NO3)2(aq) + 2NO2(g) + 2H2O(l) 

Answer 2

Because in this reaction....
Hydrogen reacts with oxygen and formed Water....