Mixtures of a weak acid and its conjugate base, such as HOAc and the OAc- ion, are called buffers. The term buffer usually means "to lessen or absorb shock." These solutions are buffers because they lessen or absorb the drastic change in pH that occurs when small amounts of acids or bases are added to water. Adding a single drop of 2 M hydrochloric acid to 100 mL of pure water changes the pH from 7 to 3.
