Solution of ammonia in water
A solution of ammonia in water turns red litmus paper blue while a solution of ammonia in methylbenzene has no effect on litmus paper.
When ammonia is dissolved in water, some of the ammonia molecules react with the polar water molecules to form ammonium hydroxide
Ammonia + water ammonium hydroxide
NH3 + H2O NH4OH
The ammonium hydroxide ionizes to form ammonium ions and hydroxide ions.
Ammmonium hydroxide ammonium ions + hydroxide ions
NH4OH NH4+ + OH-
Thus the hydroxide ions are responsible for the basic nature of ammonium hydroxide.
A solution of ammonia in methyl benzene has no effect on dry litmus paper since there are no hydroxide ions present.
Ammonia gas dissolves in methylbenzene without undergoing any change because methyl benzene is non-polar. Hence there are no hydroxide ions present ions are present in the solution of ammonia gas in methylbenzene.
As a result the solution has no effect on litmus paper.
Amphoteric oxides and Hydroxides
Metal oxides and hydroxides which react both as bases and acids are called amphoteric oxides.
Acids react with metal oxides and hydroxides to form salts and water only; the reaction is known as neutralization.
Al2O3 (s) + 6HNO3(aq) 2 AL(NO3)3 (aq) + 3H2O (L)
ZnO (s) + HNO3 (aq) Zn(NO3)2 (aq) + 3H2O(L)
PbO (s) + HNO3 (aq) PbNO3 (aq) + H2O (L)
In these reactions these metal oxides react as bases.
AL2O3 (s) + 2 NaOH (aq) 2 NaAlO2 (aq) + H2O (l)
PbO (s) + NaOH (aq) Na2PbO2 (aq) + H2O (L)
ZnO (s) + 2 NaOH (aq) NaZnO2 )(aq) + H2O (l)
In the above reactions the oxides react as acids; they react with sodium hydroxide to form a salt and water.
Lead hydroxide, zinc hydroxide and aluminium hydroxide are also amphoteric.
Pb(OH)2 (S) + 2HNO3 (aq) Pb(NO3)2 (aq) + H2O (l)
Zn(OH)2 (S) + 2HNO3 (aq) Zn(NO3)2 (aq) + H2O (L)
Al2(OH)3 (s) + 3HNO3 (aq) Al(NO3)3
+ H2O (L)
In these reactions the hydroxides are reacting as bases.