metal + Nitric acid

Under different concentration, HNO3 reacts with metals to give different gases as follows:
(1) Very dilute HNO3 (say 0.1M) :
This reacts with metals with usual to give hydrogen gas and nitrate salt, say for reaction with zinc:
Zn(s) + 2HNO3(aq) → Zn(NO3)2(aq) + H2(g)
And it can only reacts with those metals which are above H in the electrochemical series.
(2) Dilute HNO3 (say 1M) :
The main oxidizing agent is the NO3- ions and it is another type of redox reaction with half equation:
4H+(aq) + NO3-(aq) + 3e- → NO(g) + 2H2O(l)
And the NO gas formed readily combines with oxygen in air to give NO2 gas:
2NO(g) + O2(g) → 2NO2(g)
The observation is that brown fumes formed above the mouth of the test-tube since NO is colourless while NO2 is brown.
This can react with some metals below H in the electrochemical series like copper.
(3) Conc. HNO3 (say 5M) :
The main oxidizing agent is the HNO3 molecules in addition to NO3- ions and it is even a stronger oxidizing agent with half equation:
2H+(aq) + NO3-(aq) + e- → NO2(g)
The observation is that brown fumes formed immediately above the liquid surface.
This can react with some metals even lower in the electrochemical series like silver.

metal + very dilute nitric acid → metal nitrate + hydrogen
metal + dilute nitric acid → metal nitrate + nitrogen monoxide + water
metal + concentrated nitric acid → metal nitrate + nitrogen dioxide + water

easy

2NA + 2HNO3>> 2NANO3 + H2