Which term describes a solution of a weak acid (or base) and its conjugate base (or acid) that is resistant to changes in pH; works by reacting with any added acid or base to restore the equilibrium of the weak acid (or base) and neutralize the added acid or base.

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

Answer: (D)

A buffer solution is a solution that resists changes in pH because it contains a weak acid and its conjugate base (or a weak base and its conjugate acid). When you add a bit of acid, the conjugate base part neutralizes the added protons by forming the weak acid, so the pH stays largely constant. When you add a bit of base, the weak acid donates a proton to neutralize the hydroxide, forming water and the conjugate base, again minimizing pH change. The effectiveness depends on having both forms in appreciable amounts and is greatest near the acid’s pKa, often described by the Henderson–Hasselbalch relation. This combination specifically provides the capacity to neutralize added acid or base, which is why it is the best description of the scenario.