In pure water H+ (aq) and OH– (aq) ions are produced in equal numbers as a result of dissociation of water and therefore, their concentrations are also equal.
In all neutral aqueous solutions, the concentrations of H+ (aq) and OH– (aq) ions remains equal.
[H+] = [OH–]
The neutral solution is the one in which the concentrations of H+ and OH– ions are equal.
Acids furnish H+ (aq) ions in their solutions resulting in increase in their concentration. Thus, in acidic solutions
[H+] > [OH–]
[H+] > 1.0 × 10–7 mol L–1
The acidic solution is the one in which the concentration of H+ (aq) is greater than that of OH– (aq) ions.
The ionic product of water Kw is constant at a given temperature. It can remain so only if the concentration of OH– (aq) ions decreases.
[OH–] < 10–7 mol L–1
Bases furnish OH– (aq) ions in their solutions. This results in an increase in their concentration. Therefore, in basic solution
[OH–] > [H+]
[OH–] > 1.0 × 10–7 mol L–1
The basic solution is the one in which the concentration of H+ (aq) ions is smaller than that of OH–1 (aq) ions. Here also, because of constancy of ionic product of water Kw, the concentration of H+ (aq) decreases.
[H+] < 1.0 × 10–7 mol L–1