what would the product be of HNO3 is heated
It might change phases from liquid to gas. If you drive off all of the water you would be left with N2O5 which is where HNO3 comes from in the first place.
N2O5 + H2O ==> 2HNO3 ==> H2O + N2O5
N2O5 + H2O ==> 2HNO3 ==> H2O + N2O5