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 rest 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.

Answer 1

The following reaction takes place in the test tube

i) When Aluminium is treated with dilute hydrochloric acid.

$2Al\quad +\quad 6HCl\quad \rightarrow \quad 2Al{ Cl }_{ 3 }\quad +\quad 3{ H }_{ 2 }$

ii) When Aluminium is treated with dilute sodium hydroxide solution.

$2Al\quad +\quad 2NaOH\quad +\quad 2{ H }_{ 2 }O\quad \rightarrow \quad 2NaAl{ O }_{ 2 }\quad +\quad 3{ H }_{ 2 }$

In both the cases the Hydrogen atoms are evolved. So, when a burning match stick is brought near the test tube we hear a pop sound.

Aluminium does not react with concentrated nitric acid. A "layer" of "Aluminium Oxide" is formed, when aluminium reacts with the nitric acid because "nitric acid" is an oxidising agent. Due to the "layer" of “Aluminium oxide”, nitric acid cannot react with the “inner aluminium metal”.