Which system, consisting of a weak acid or weak base and its salt, resists changes in pH?

The system that resists changes in pH, consisting of a weak acid or weak base and its salt, is known as a buffer. Buffers work by being able to absorb excess hydrogen ions (H+) or hydroxide ions (OH-) that may be introduced into a solution, thus stabilizing the pH level.

When an acid is added to a buffer solution, the weak base component of the buffer will react with the added H+ ions, minimizing the impact on pH. Conversely, if a base is introduced, the weak acid component will donate H+ ions to neutralize the added OH- ions, again helping to maintain pH stability.

This property makes buffers essential in biological systems and laboratory settings, where it is crucial to maintain a stable pH for enzyme activity and other chemical reactions.