(a) When cone. HNO, is heated, it decomposes to give brown coloured pungent smelling gas nitrogen dioxide (NO2 ).
(b) When. ”cone. HNO3 is heated in the presence of copper, brown coloured, pungent smelling NO2 is formed alongwith blue coloured copper nitrate.
Cu + 4HNO(cone.)→ Cu(NO3 )2 + 2H2O + 2NO2