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Question
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 solution in a test tube and on bringing a burning matchstick near the mouth of the test tube, a pop sound indicates the evolution of hydrogen gas. The same activity when performed with concentrated nitric acid, reaction doesn’t proceed. Explain the reason.
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Solution
Al being amphoteric dissolves both in acids and alkalies evolving H2 gas which bums with a pop sound.
\[\ce{2Al + 6HCl -> 2AlCl3 + 3H2}\]
\[\ce{2Al + 2NaOH + 2H2O -> \underset{Sod meta-aluminate}{2NaAlO2} + 3H2}\]
With conic. HNO3, Al becomes passive and the reaction does not proceed. This passivity is due to the formation of a thin protective layer of its oxide (Al2O3) on the surface of the metal which prevents further action.
\[\ce{2Al + 6HNO3 -> Al2O3 + 6NO2 + 3H2O}\]
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