Ions of weak acids or bases react with water, making salt solutions acidic or basic.
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When a salt dissolves, its ions can react with water, a process called hydrolysis that shifts the pH. Cations derived from weak bases, such as NH4+, donate protons to water and make the solution acidic, while anions derived from weak acids, such as acetate, accept protons and make it basic. Small, highly charged metal ions such as Al3+ are acidic because the hydrated ion donates a proton from its coordinated water molecules. Ions derived from strong acids or strong bases, such as Na+ and Cl-, do not hydrolyse and have no effect on pH. The extent of hydrolysis is governed by Kh = Kw/Ka or Kw/Kb.
Key ideas: Ions of weak acids or bases react with water. Cations of weak bases (NH4+) give acidic solutions; anions of weak acids (CH3COO-, CN-, CO3 2-) give basic ones; ions of strong acids and bases (Na+, Cl-, K+, NO3-) do not hydrolyze. Small, highly charged metal cations such as Al3+ and Fe3+ are acidic via hydrated ion ionization.
Common mistakes and connections: Typical mistake: calling a salt neutral because its formula has no H or OH. When both ions hydrolyze (NH4CN), compare Ka of the cation with Kb of the anion: the larger wins. Hydrolysis is simply Bronsted behavior of conjugate ions, and relates to buffers and equivalence-point pH.
Hydrolysis shifts salt pH: acidic cations (Al3+, Fe3+, NH4+) lower it, basic anions (acetate, carbonate) raise it, and Na+ with Cl- do nothing. The interactive lesson shows this as a live animation or chart.
A solution of NH4Cl is acidic because NH4+ hydrolyses, while a solution of sodium acetate is basic because acetate hydrolyses.
Ions of weak acids or bases react with water, making salt solutions acidic or basic.
Hydrolysis of Salts is a intermediate topic in Acids, Bases & Electrochemistry. It builds on the vocabulary of beginner topics in this area.
Study these earlier topics in Acids, Bases & Electrochemistry first: pH of Weak Acid Solutions, Polyprotic Acids, Acid Strength and Structure.
The next topics in Acids, Bases & Electrochemistry are pH of Salt Solutions and Acid-Base Neutralization.
Yes. The interactive lesson includes a self-check question on Hydrolysis of Salts, and the Acids, Bases & Electrochemistry quiz draws on every topic in this area.
Hydrolysis of Salts is topic 19 of 105 in Acids, Bases & Electrochemistry.