Nitric acid is a strong oxidising agent. During reaction, it oxidise to produced hydrogen gas and convert it into water.
Reaction of HNO3 with Mn.
Mn reacts with dilute nitric acid to give manganese nitrate and hydrogen gas.
Mn + 2HNO 3 → Mn ( NO 3 ) 2 + H 2
Reaction of HNO3 with Mg.
Mg + 2 HNO3 → Mg(NO3)2 + H2