Pure water is a weak electrolyte and neutral in nature, i.e., H+ ion concentration is exactly equal to OH" ion concentration
[H+] = [OH-]
When this condition is disturbed by decreasing the concentration of either of the two ions, the neutral nature changes into acidic or basic. When [H+] > [OH-], the water becomes acidic and when [H+] < [OH-], the water acquires basic nature. This is exactly the change which occurs during the phenomenon known as salt hydrolysis. It is defined as a reaction in which the cation or anion or both of a salt react with water to produce acidity or alkalinity.
Salts are strong electrolytes. When dissolved in water, they dissociate almost completely into ions. In some salts, cations are more reactive in comparison to anions and these react with water to produce H+ ions. Thus, the solution acquires acidic nature.
M+ + H2O ↔ MOH + H+
In other salts, anions may be more reactive in comparison to cations and these react with water to produce OH- ions. Thus, the solution becomes basic.
A- + H2O ↔ HA + OH-
The process of salt hydrolysis is actually the reverse of neutralization.
Salt + Water ↔ Acid + Base
If acid is stronger than base, the solution is acidic and in case base is stronger than acid, the solution is alkaline. When both the acid and the base are either strong or weak, the solution is generally neutral in nature.
As the nature of the cation or the anion of the salt determines whether its solution will be acidic or basic, it is proper to divide the salts into four categories.
(i) Salt of a strong acid and a weak base.
Examples: FeCl3, CuCl2, AlCl3, NH4Cl, CuSO4, etc.
(ii) Salt of a strong base and a weak acid.
Examples: CH3COONa, NaCN, NaHCO3, Na2CO3, etc.
(iii) Salt of a weak acid and a weak base.
Examples: CH3COONH4, (NH4)2CO3, NH4HCO3, etc.
(iv) Salt of a strong acid and a strong base.
Examples: NaCl, K2SO4, NaNO3, NaBr, etc.