Salt hydrolysis: why some salts change pH
Learn why some salt solutions are acidic, some basic, and some neutral — and how to predict which by looking at the parent acid and base.
Quick answer
Salt hydrolysis is the reaction of a salt's ion with water to produce H⁺ or OH⁻. Ions from weak acids/bases hydrolyze; ions from strong acids/bases are spectators.
Example: NH₄Cl: NH₄⁺ + H₂O ⇌ NH₃ + H₃O⁺ (acidic, Cl⁻ is a spectator). NaCH₃COO: CH₃COO⁻ + H₂O ⇌ CH₃COOH + OH⁻ (basic, Na⁺ is a spectator). NaCl: neither ion hydrolyzes (neutral).
What you will learn
- Define salt hydrolysis as an ion reacting with water.
- Predict acidic, basic, or neutral behavior from the parent acid and base strengths.
- Write hydrolysis equations for cations and anions.
- Explain metal-ion hydrolysis for highly charged cations.
Why some salts change pH
Dissolve table salt and the pH stays 7. Dissolve sodium acetate and the solution turns basic. Dissolve ammonium chloride and it turns acidic. All three are 'salts' — so why the difference?
The answer is hydrolysis: an ion reacting with water to generate H⁺ or OH⁻. The rule is simple — ions from weak partners hydrolyze; ions from strong partners do not (they are spectators). Every salt falls into one of four cases based on its parent acid and base.
The four cases
Case 1 — strong acid + strong base → neutral. NaCl: Na⁺ (from strong base NaOH) and Cl⁻ (from strong HCl) are both spectators. pH = 7.
Case 2 — weak acid + strong base → basic. Sodium acetate: Na⁺ is a spectator, but acetate hydrolyzes:
producing OH⁻ (Kb = 5.6 × 10⁻¹⁰). pH > 7.
Case 3 — strong acid + weak base → acidic. Ammonium chloride: Cl⁻ is a spectator, but ammonium hydrolyzes:
producing H₃O⁺ (Ka = 5.6 × 10⁻¹⁰). pH < 7.
Case 4 — weak acid + weak base → compare Ka vs Kb. Whichever ion's constant is larger wins; if they are close, the solution is approximately neutral.
Metal ions hydrolyze too
Small, highly charged metal cations — Al³⁺, Fe³⁺, Cr³⁺ — hydrolyze even though they are not Brønsted acids in the usual sense. As Lewis acids, they polarize coordinated water molecules so strongly that a proton is released:
This is why AlCl₃ solutions are acidic (pH ~3) and why aluminum salts are used in water treatment to coagulate impurities. Large singly-charged ions (Na⁺, K⁺) are too weakly polarizing to matter.
Putting numbers on it
Hydrolysis is quantitative: for an anion, Kh = Kw/Ka(parent acid); for a cation, Kh = Kw/Kb(parent base). For 0.10 M sodium acetate: [OH⁻] ≈ √(Kh·C) = √(5.6 × 10⁻¹⁰ × 0.10) = 7.5 × 10⁻⁶ M → pOH 5.13 → pH 8.87. Basic, as predicted — and now with a number attached.
This same math runs backward into buffer design: every buffer's base partner is a hydrolyzing anion doing exactly this job.
Common mistakes
Only salts of strong acids and strong bases (NaCl, KNO₃) are neutral. Salts containing the ion of a weak acid or weak base hydrolyze and shift the pH — NaCH₃COO is basic, NH₄Cl is acidic.
Dissolving (dissociation into ions) happens for all soluble salts. Hydrolysis is the second step: one of those ions reacting with water. NaCl dissolves but does not hydrolyze; NH₄Cl dissolves and then NH₄⁺ hydrolyzes.
Hydrolysis equilibria follow the usual √-type math, so pH changes only slowly with concentration (about 0.5 unit per tenfold dilution). The direction (acidic/basic) never changes with concentration — only the magnitude.
Real-world applications
Predicting solution pH: knowing whether a salt solution is acidic, basic, or neutral.
Buffer preparation: salts of weak acids are buffer components.
Soap chemistry: soap solutions are basic because of hydrolysis.
Fertilizer chemistry: ammonium salts acidify soil through hydrolysis.
Frequently asked questions
How do I predict whether a salt solution is acidic or basic?
Look at the parents: strong acid + strong base → neutral (NaCl); weak acid + strong base → basic (NaCH₃COO); strong acid + weak base → acidic (NH₄Cl); weak acid + weak base → compare Ka and Kb (usually near neutral).
Why does only one ion hydrolyze in salts like NH₄Cl?
The ion from the strong partner is a spectator (too weak to react with water); the ion from the weak partner hydrolyzes. In NH₄Cl, Cl⁻ (from strong HCl) does nothing while NH₄⁺ (from weak NH₃) donates a proton — net acidic.
Can metal ions hydrolyze too?
Yes — small, highly charged metal ions (Al³⁺, Fe³⁺, Cr³⁺) are Lewis acids that polarize water molecules and release H⁺. AlCl₃ solutions are distinctly acidic for this reason, even though Al(OH)₃ is a 'base.'
What about a salt of a weak acid AND a weak base?
Compare Ka of the cation's conjugate acid with Kb of the anion's conjugate base. For NH₄CH₃COO: Ka(NH₄⁺) = 5.6 × 10⁻¹⁰ vs Kb(CH₃COO⁻) = 5.6 × 10⁻¹⁰ — essentially equal, so the solution is ~neutral. Whichever constant is larger tips the pH that way.