Aluminium dissovles in mineral acids and aqueous alkalies and thus shows amphoteric character. A place of aluminum foil is treated with dilute hydrochloric acid or dilute sodium hydroxide solution in a test tube and on bringing a burning match stick near the mouth of the test tube, a pop sound indicates the evolution of hydrogen gas. The same activity when performed with concentrated with concentrated nitric acid. reaction doesn't proceed. Expalin the reason.

Explanation

Al being amphoteric dissolves both in acids and alkalies evolving `H_(2)` gas which burns with a pop south
`2Al + 6HCl rarr AlCl_(3) + 3H_(2)`
`2Al + 2NaOH + 2H_(2)O rarr underset(" Sod. Meta-aluminate ")(2 NaAlO_(2)) +3H_(2)`
With conc. `HNO_(3)`, Al becomes passive and the reaction does not proceed. This passive is due to fromation of a thin pratective layer of its oxide `(Al_(2)O_(3))` on the surface of the metal which prevents further action.
`2Al + 6HNO_(3) rarr Al_(2)O_(3) + 6NO_(2) + 3H_(2)O`.