Ncert Exercises & Solutions

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.

Aluminium being amphoteric in nature dissolves both in acids and alkalies evolving H₂ gas which burns with a pop sound.

2 Al + 6 HCl \to 2 {AlCl}_{3} + 3 H_{2}

2 Al + NaOH + 2 H_{2} O \to \underset{Sodium meta aluminate}{2 {NaAlO}_{2}} + 3 H_{2}

But when Al is treated with conc. HNO₃, a thin protective layer of Al₂O₃ is formed on its surface which prevents further reaction.

2 Al + 6 {HNO}_{3} \to {Al}_{2} O_{3} + 6 {NO}_{2} + 3 H_{2} O

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