Hydrogen gas is not evolved when a metal reacts with nitric acid because nitric acid (HNO3) is a strong oxidizing agent. It oxidizes the hydrogen gas produced to water while itself getting reduced to various nitrogen oxides (N2O, NO, NO2).
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When a metal reacts with nitric acid, hydrogen gas is typically not evolved due to the oxidizing nature of nitric acid. Nitric acid is a strong oxidizing agent and can oxidize hydrogen ions (H⁺) produced in the reaction, preventing the formation of hydrogen gas. Instead, nitrogen oxides (NOx) are often produced as byproducts. The specific reaction depends on the metal involved, and in some cases, a layer of oxide or nitrate is formed on the metal surface, acting as a protective barrier and further inhibiting the release of hydrogen gas. This distinctive behavior sets nitric acid apart from other acids in metal reactions.