Hydrolysis of salts and the medium of the solution
Hydrolysis (Greek «hydro» — water, «lysis» — breakdown) is the reaction of salt ions with water to form a weak electrolyte. A salt of a strong acid and a weak base (NH₄Cl, ZnCl₂, Cu(NO₃)₂) hydrolyses by the cation: H⁺ ions accumulate and the medium is acidic (pH < 7), for example Cu²⁺ + H₂O ⇌ CuOH⁺ + H⁺. A salt of a strong base and a weak acid (CH₃COONa, Na₂CO₃, K₂S) hydrolyses by the anion: OH⁻ accumulates and the medium is alkaline (pH > 7), for example CO₃²⁻ + H₂O ⇌ HCO₃⁻ + OH⁻. A salt of a weak base and a weak acid (CH₃COONH₄) hydrolyses by both ions and the medium is roughly neutral; some are broken down by water irreversibly (Al₂S₃ + 6H₂O → 2Al(OH)₃↓ + 3H₂S↑). Salts of a strong base and a strong acid (NaCl, K₂SO₄, KNO₃) and insoluble salts do not hydrolyse and the medium is neutral. The medium is expressed by pH = –lg[H⁺]: pH = 7 neutral, pH < 7 acidic, pH > 7 alkaline (blood pH ≈ 7.4, stomach 1.5–2). Hydrolysis strengthens on heating and dilution and also speeds up when a substance binding the H⁺ or OH⁻ formed is added; the medium is detected with litmus, phenolphthalein and methyl orange.
Teacher demonstration: test the medium of NaCl, Na₂CO₃ and NH₄Cl solutions with litmus paper or universal indicator. Students record observations in a table and compare them with hydrolysis equations (chemicals are handled only with the teacher present).