What is a buffer solution composed of?

A buffer solution is composed of a mixture that typically includes a weak acid and its conjugate base or a weak base and its conjugate acid. This combination is crucial because it allows the solution to resist changes in pH when small amounts of acid or base are added. The weak acid can neutralize added bases, while its conjugate base can neutralize added acids, thus maintaining a relatively stable pH.

In the case of a weak acid and its conjugate base, the weak acid partially dissociates in solution, providing some hydrogen ions (H⁺), while the conjugate base can react with H⁺ to minimize changes in pH. Similarly, if you start with a weak base and its conjugate acid, the weak base can neutralize any added acids, and the conjugate acid can neutralize added bases.

This property is fundamental in various chemical and biological systems, where maintaining a stable pH is critical. The other options involve strong acids or bases, which do not provide the same kind of effective buffering ability since they fully dissociate in solution and do not generate the necessary equilibrium to resist pH changes the way buffers do.