Advanced Placement (AP) Chemistry Practice Exam 2025 - Free AP Chemistry Practice Questions and Study Guide

A buffer solution is characterized by the ability to maintain a relatively stable pH when small amounts of acid or base are added to it. This stability arises from the presence of a weak acid paired with its conjugate base or a weak base with its conjugate acid. The components in a buffer solution work together to neutralize added acids or bases, thereby minimizing shifts in pH.

For instance, in an acidic buffer, the weak acid can donate protons (H⁺) when a base is added, while the conjugate base can accept protons when acid is introduced. This dual capability allows the buffer system to effectively resist changes in pH across a relevant range, providing significant control over the solution’s acidity or basicity.

In contrast to the other options, a buffer does not have to exclusively be acidic or basic, nor does it only operate within low concentrations; it can function across various concentrations. Additionally, a buffer does not need the presence of a strong acid for its effectiveness, as its primary mechanism relies on the weak acid-base pairs rather than strong acids or bases. This distinctive composition is what makes option C the defining characteristic of a buffer solution.