Why can a salt solution be acidic or basic after neutralization?

A salt solution can be acidic or basic after neutralization due to a process called hydrolysis. While neutralization is often thought of as the cancellation of acid and base properties, the resulting salt solution's pH depends on the relative strengths of the original reactants.12

In the Brønsted–Lowry concept, hydrolysis occurs because ions derived from weak acids or weak bases retain acidic or basic properties in water. Specifically:2

• Acidic solutions: Salts derived from a weak base and a strong acid often produce acidic solutions. For example, the ammonium ion ($N{H}_4^{+}$) from ammonium chloride acts as an acid, increasing the concentration of hydronium ions ($H_3{O}^{+}$) in the solution. Additionally, salts of certain metals with multiply charged cations, such as aluminum, become acidic because their hydrated ions transfer protons to water molecules.2
• Basic solutions: Salts derived from a weak acid and a strong base often produce basic solutions. For instance, the anion from sodium acetate acts as a base, reacting with water to produce hydroxide ions ($O{H}^{-}$), which increases the solution's basicity.2

If one reactant is present in significant excess, the resulting solution will naturally reflect the properties of that excess reactant.1