• # question_answer 78)   Aluminium dissolves in mineral acids and aqueous alkalies and thus shows amphoteric character. A piece of aluminium foil is treated with dilute hydrochloric acid or dilute sodium hydroxide in a test tube and on bringing a burning matchstick near the mouth of the test tube, a pop sound indicate the evolution of hydrogen gas. The same activity when performed with concentrate nitric acid, reaction does not proceed. Explain the reason.

Concentrated nitric acid renders aluminium passive (inactive) by forming a protective layer of oxide $(A{{l}_{2}}{{O}_{3}})$on the surface. Thus, hydrogen gas is not evolved when aluminium is treated with concentrated$HN{{O}_{3}}$.