Salt Hydrolysis

The aqueous solutions of certain salts also behave as acids or bases. They do so because of the hydrolysis of the cation or anion or both. Depending on the behaviour towards hydrolysis there are four different types of salts:

  1. Salt of strong acid + strong base (HCl + NaOH) NaCl
  2. Salt of strong acid + weak base (HCl + NH4OH) NH4Cl
  3. Salt of weak acid + strong base (CH3COOH + NaOH) CH3COONa
  4. Salt of weak Acid + weak Base (CH3COOH + NH4OH) CH3COONH4

Salt of strong acid + strong base

The cations of the strong bases and the anions of the strong acids do not get hydrolyzed. Therefore the salts of this category do not show any acid-base behavior and are neutral.

Salt of strong acid + weak base

The salts of this type dissociate in aqueous solutions to give a cation of a weak base and the anion belonging to strong acid. For example,

NH4Cl (aq) → NH4+ (aq) + Cl- (aq)

The anion does not get hydrolyzed but the cation does get hydrolyzed.

NH4+ (aq) + H2O (l) → NH4OH + H+ (aq)

Since it generates H+ (aq) ions, the solution is acidic in nature.

Salt of weak acid and strong base

The salts of this type dissociate in aqueous solutions to give a anion of a weak acid and the cation belonging to strong base. For example,

CH3COONa (aq) → Na+ (aq) + CH3COO- (aq)

The cation does not get hydrolyzed but the anion does get hydrolyzed.

CH3COO- (aq) + H2O (l) → CH3COOH (aq) + OH- (aq)

Since it generates hydroxyl ions the solution is basic in nature.

Salt of weak Acid and weak Base

The salts of this type dissociate in aqueous solutions to give a anion of a weak acid and the cation belonging to a weak base. For example, 

CH3COONH4 (aq) ⇌ NH4+ (aq) + CH3COO- (aq)

In this case both the cation as well as the anion would undergo hydrolysis and the nature of the solution, whether acidic, basic or neutral would depend on the relative strength of the weak acid and the weak base.