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Question

Aluminum 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 reacts with both acid and base and hence, it is an amphoteric metal.
  • On reacting with HCl and NaOH, Al liberates H2 gas which burns in air with a pop sound.

2Al(s)+2NaOH(aq)+6H2O(l)2Na+[Al(OH)4)]+3H2(g)


2Al(s)+6HCl(aq)2 Al3+(aq)+6Cl+3H2(g)i.e 2AlCl3+3H2

While HNO3 is a very strong oxidizing agent and it oxidises Al to Al2O3 (Aluminium oxide) forming a thin layer of Al2O3 on the surface of Al.

2Al(s)+6HNO3(aq.)Al2O3(s)+6NO2(g)+3H2O(l)
  • This layer protects the further reaction of Al with HNO3 and thus no H2 gas is liberated.

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