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 does not proceed. Explain the reason.

Answers (1)

Al when dissolved in acids and alkalis and thus is amphoteric and thus gives out H2 gas which can be tested as it burns with a pop sound.

2Al + 6HCl \rightarrow 2AlCl_{3} + 3H_{2}

2Al + 2NaOH + 2H_{2}O \rightarrow 2NaAlO_{2} + 3H_{2}

When the nitric acid becomes passive, the reaction stops, and the thin layer of aluminium oxide ceases the reaction.

2Al + 6HNO_3 \rightarrow Al_{2}O_{3} + 6NO_{2} + 3H_{2}O

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