Question

Is ammonia solution acidic or basic?

Answer 1

In order to find out the nature of ammonia, its reaction and behavior in an aqueous medium can be studied. The ions released by ammonia in water and its reaction with other acids and bases can throw some light on its nature.

Complete answer:
A chemical compound can be called an acid as well as a base depending upon the theory used to categorize it.
According to the Arrhenius Acid-base theory, any chemical compound that releases hydrogen ions (or protons) in aqueous solutions is called an acid and the one that releases hydroxide ions in aqueous solution is called a base. The strength of an Arrhenius acid is determined by the amount of hydrogen ions ${H^ + }$ released and that of an Arrhenius base is determined by the amount of hydroxide ions $O{H^ - }$ released by it.
But Ammonia is a weak Arrhenius base that does not dissociate on its own but when present in an aqueous medium, it releases ammonium and hydroxide ions. The reaction is in equilibrium and a very small amount of $O{H^ - }$ ions is released.
$N{H_3} + {H_2}O \rightleftharpoons N{H_4}^ + + O{H^ - }$
According to the Bronsted-Lowry acid base theory, the compounds that donate or release hydrogen ions are said to be acidic and the ones that accept the hydrogen ions are considered basic in nature.
Ammonia is an amphoteric compound according to the Bronsted-Lowry acid base theory as it is capable of both accepting as well donating or releasing hydrogen ions. It behaves like an acid in presence of a stronger base and a base in the presence of a stronger acid.
$N{H_3} \rightleftharpoons {H^ + } + N{H_2}^ - {\text{ (behaves as acid)}}$
$N{H_3} + {H^ + } \rightleftharpoons N{H_4}^ + {\text{ (behaves as base)}}$
Thus ammonia is a weak base in water and an amphoteric compound according to the Bronsted-Lowry acid base theory.

Note:
An amphoteric compound is capable of acting as an acid as well as a base. Ammonia is generally basic in nature (according to the Arrhenius theory as well as the Lewis acid-base theory) but can behave as an acid only in the presence of strong organic acids that can extract a proton out of it.