When metals are treated with hno3 …

Normally when metals are teated with acids, a corresponding salt is produced along with the liberation of hydrogen gas. i.e. Metal + Acid* ----> Salt + Hydrogen (g) However, nitric acid (HNO_3) being a strong oxidizing agent** oxidizes the hydrogen gas (produced when it reacts with metals) to give water and itself gets reduced to an oxide of nitrogen which combines with the metal. i.e. Metals*** + Nitric Acid ----> (Nitrogenous) Salt + Water However, Manganese and Magnesium react with dilute nitric acid to give hydrogen gas. i.e. Mg or Mn + Nitric Acid ---> Salt + Hydrogen (g) ____________________________________________________________________________________________ Notes: * Except Nitric Acid ** (here) A substance which loses oxygen and adds it to another substance *** Except Mn or Mg

It is because HNO3 is a strong oxidising agent. It oxidise the hydrogen to form water. But in some cases like Mg and Mn react with very dilute nitric acid (HNO3 ) to form hydrogen..