Which term describes a solution that resists changes in pH when small amounts of acid or base are added?

• A. Buffer Solution
• B. Polyprotic acid
• C. Amphiprotic species
• D. Hydrolysis reaction

Answer: (A)

A solution that resists changes in pH when small amounts of acid or base are added is a buffer. This works because a buffer contains a weak acid and its conjugate base (or a weak base and its conjugate acid). If acid is added, the conjugate base neutralizes some of the extra H+ by forming more of the weak acid, so the pH doesn’t drop much. If base is added, the weak acid donates a proton to neutralize the OH−, forming more of the conjugate base and keeping the pH from rising significantly. The Henderson–Hasselbalch relationship pH = pKa + log([A−]/[HA]) shows that when the ratio of base form to acid form stays near the right balance, the pH remains fairly stable. Buffer capacity increases with higher concentrations of both species. Examples include acetic acid with acetate or ammonia with ammonium; these pairs illustrate the ongoing neutralization that dampens pH changes. The other terms describe related ideas (polyprotic acids can donate multiple protons, amphiprotic species can act as both acid and base in some contexts, and hydrolysis is a reaction with water) but do not define a solution that resists pH changes as precisely as a true buffer does.