What is a buffer solution?

A buffer solution is defined as a solution that resists changes in pH upon the addition of small amounts of acid or base. This ability stems from the presence of a weak acid and its conjugate base (or a weak base and its conjugate acid) in equilibrium. When an acid is added to the buffer, the weak base component reacts with the added H⁺ ions to minimize the change in pH. Conversely, if a base is added, the weak acid component donates H⁺ ions to counteract the increase in pH. This property makes buffer solutions especially important in biological and chemical systems, where maintaining a stable pH is crucial for proper functioning.

The other options do not accurately capture the defining characteristics of buffer solutions. For instance, a solution that drastically changes pH with small additions of acid or base does not have buffering capacity, and thus does not qualify as a buffer. A solution with a fixed pH does not allow for pH changes, which is contrary to the dynamic equilibrium present in buffer solutions. Finally, a solution that only contains acids lacks the necessary components (conjugate bases) that are crucial for buffering action.