Why Is a Salt Solution Acidic, Basic, or Neutral? (Hydrolysis) (HL)

A salt solution is acidic, basic, or neutral depending on whether its ions react with water and alter the relative concentrations of $\text{H}^+$ and $\text{OH}^-$. This process is called salt hydrolysis.

When a salt dissolves, it dissociates into ions. An ion derived from a strong acid or strong base normally does not hydrolyse appreciably, whereas the conjugate of a weak acid or weak base reacts with water.

These reactions are proton-transfer equilibria governed by conjugate acid-base strength. A strong acid has an extremely weak conjugate base, so chloride does not compete effectively with water for protons. Likewise, sodium ions from the strong base sodium hydroxide do not act as Brønsted-Lowry acids. Therefore, the ion originating from the weak parent species mainly controls the pH. This is AHL Reactivity 3.1.12.

For ammonium chloride, $\text{Cl}^-$ is the conjugate base of the strong acid $\text{HCl}$, so it has negligible basicity. However, ammonium ions undergo hydrolysis:

$$\text{NH}_4^+(aq)+\text{H}_2\text{O}(l)\rightleftharpoons\text{NH}_3(aq)+\text{H}_3\text{O}^+(aq)$$

The production of $\text{H}_3\text{O}^+$ makes the solution acidic.

For sodium ethanoate, $\text{Na}^+$ does not hydrolyse appreciably, but ethanoate ions accept protons:

$$\text{CH}_3\text{COO}^-(aq)+\text{H}_2\text{O}(l)\rightleftharpoons\text{CH}_3\text{COOH}(aq)+\text{OH}^-(aq)$$

The production of $\text{OH}^-$ makes the solution basic. If both ions hydrolyse, compare $K_a$ for the acidic cation with $K_b$ for the basic anion: the larger value determines the solution's character; similar values produce an approximately neutral solution.

A common misconception is that every salt solution is neutral. In an IB explain question, identify each ion's parent acid or base, write the relevant hydrolysis equation, and link the formation of $\text{H}_3\text{O}^+$ or $\text{OH}^-$ to pH. Remember that neutral means $[{}^{}$; pH 7 applies only at 298 K.