Hydrolysis is the decomposition of a chemical compound by the addition of water molecules or ions. Usually it is being termed when there is an existing conjugate acid or base in the solution. One factor that should be considered for an ion to hydrolyze is its conjugate pair. An ion generally can be hydrolyze it its conjugate pair came from a weak acid or base otherwise the reaction will not be successful. Let me show you an example.
The dissociation of ammonium chloride in the solution of water.
`NH4Cl (aq) -> NH4^(+) + Cl^(-)`
`NH4^(+)` ion is a conjugate acid of a weak base NH3
`Cl^(-1)` chloride ion is a conjugate base of a strong acid HCl
Based on the criteria said above, only `NH4^(+)` will be hydrolyzed. The hydrolysis reaction can be written as:
`NH4^(+) + H2O <=> NH3 + H3O^(+)`
`Cl^(-)` ions cannot be hydrolyzed.
Another example: The dissociation of sodium acetate in the solution.
`NaCH3CO2 -> Na^(+) + CH3CO2^(-)`
Only the acetate ion, `CH3COO^(-)` can be hydrolyzed.
The reaction can be written as:
`CH3COO^(-) + H2O <=> CH3COOH + OH^(-)`
Another factor, some transition metals ions (d block elements) can be hydrolyzed since they can alter the pH of the solution upon the reaction with water.
Some examples are `Fe^(3+) and Cu^(2+)` ions.