Question

Which of the following metallic acids exhibit amphoteric nature?
A. ${\text{CaO}}$
B. ${\text{N}}{{\text{a}}_2}{\text{O}}$
C.${\text{BaO}}$
D. ${\text{A}}{{\text{l}}_2}{{\text{O}}_3}$

Answer 1

To answer this question we should know what are metallic, non-metallic and amphoteric oxides are. The nature of the oxide is decided based on the product of hydrolysis. Metallic oxides behave as a base. Non-metallic oxides behave as acid. Amphoteric oxides behave as acid as well as a base.
Complete step-by-step answer:
Oxides are of three types: Metallic, Non-metallic and Amphoteric. Metallic and amphoteric oxides have metals and non-metallic oxides have non-metal. The metal oxide on dissolving in water produces base, so they are basic. The non-metal oxide on dissolving in water produces acid, so they are acidic.

Some metallic oxides generally do not react with water but react with base as well as acid, are known as amphoteric oxides. Calcium oxide ${\text{CaO}}$reacts with water to give base calcium hydroxide, so it is basic.
${\text{CaO}}\,{\text{ + }}\,{{\text{H}}_{\text{2}}}{\text{O}} \to \,{\text{Ca(OH}}{{\text{)}}_2}$
Sodium oxide ${\text{N}}{{\text{a}}_2}{\text{O}}$reacts with water to give base sodium hydroxide, so it is basic.
${\text{N}}{{\text{a}}_2}{\text{O}}\,{\text{ + }}\,{{\text{H}}_{\text{2}}}{\text{O}} \to \,2\,{\text{NaOH}}$
Barium oxide ${\text{BaO}}$reacts with water to give base barium hydroxide, so it is basic.
${\text{BaO}}\,{\text{ + }}\,{{\text{H}}_{\text{2}}}{\text{O}} \to \,{\text{Ba(OH}}{{\text{)}}_2}$
Aluminium oxide${\text{A}}{{\text{l}}_2}{{\text{O}}_3}$ does not react with water because it is insoluble in water.
Aluminium oxide${\text{A}}{{\text{l}}_2}{{\text{O}}_3}$reacts with acid to give aluminium chloride and water.
${\text{A}}{{\text{l}}_{\text{2}}}{{\text{O}}_{\text{3}}}\,{\text{ + }}\,6\,{\text{HCl}}\, \to {\text{2AlC}}{{\text{l}}_{\text{3}}}\,{\text{ + 3}}\,{{\text{H}}_{\text{2}}}{\text{O}}$
Aluminium oxide${\text{A}}{{\text{l}}_2}{{\text{O}}_3}$reacts with base in presence of water to give hydroxide of aluminium.
${\text{A}}{{\text{l}}_{\text{2}}}{{\text{O}}_{\text{3}}}\,{\text{ + }}\,2\,{\text{NaOH}}\,\, + {\text{3}}\,{{\text{H}}_{\text{2}}}{\text{O}} \to {\text{2NaAl(OH}}{{\text{)}}_4}$
Aluminium oxide ${\text{A}}{{\text{l}}_2}{{\text{O}}_3}$reacts with acid as well as base so, it is amphoteric.
So, ${\text{A}}{{\text{l}}_2}{{\text{O}}_3}$ exhibits amphoteric nature.

Therefore, option (D) ${\text{A}}{{\text{l}}_2}{{\text{O}}_3}$ are correct.

Note: Amphoterism depends upon the oxidation state of the metal. As the oxidation state increases the amphoterism increases. Because of increasing positive charge the polarising power of the metal increases. The non-metal oxides are acidic. The metallic oxides are basic. Amphoteric oxides are both acidic as well as basic.