Hydrolysis of salts

Pure water is a very weak electrolyte: H₂O ⇄ H⁺ + OH⁻, with [H⁺] = [OH⁻], so it is neutral. When a salt dissolves and its ions combine with the ions of water to form a weak electrolyte, the process is called hydrolysis. Salts of a strong base and a strong acid (KNO₃, NaCl, Na₂SO₄) do not hydrolyse and the medium is neutral. In a salt of a weak base and a strong acid (Al(NO₃)₃, CuSO₄, NH₄Cl) the cation hydrolyses: Al³⁺ + H₂O ⇄ AlOH²⁺ + H⁺, and the excess H⁺ makes the medium acidic. In a salt of a strong base and a weak acid (Na₂CO₃, K₂S) the anion hydrolyses: CO₃²⁻ + H₂O ⇄ HCO₃⁻ + OH⁻, and the medium is alkaline. A salt of a weak base and a weak acid (Al₂S₃) hydrolyses completely: Al₂S₃ + 6H₂O → 2Al(OH)₃↓ + 3H₂S↑, so it cannot be obtained from solution. Heating and dilution strengthen hydrolysis; to keep a solution from hydrolysing it is best stored cold and concentrated.