Explain why adding small amounts of acid or base hardly changes the pH of an acidic buffer solution.

An acidic buffer solution is made up of a weak acid (e.g. ethanoic acid) and a salt containing its conjugate base (e.g. sodium ethanoate). It resists pH change via two mechanisms: The fact that the acid is weak and hardly dissociates means that when H⁺ ions are added, most react with conjugate base ions and get trapped as weak acid molecules. When OH^- ions are added, they are consumed by reaction with the acid molecules to make H₂O and more of the conjugate base's salt. In the first case, dissociated protons are consumed to make more weak acid that hardly dissociates (increasing acidity much less than otherwise), and in the second case, hydroxide ions only act to lower the concentration of weak acid, something which has a lesser effect on acidity than simply adding strong base to a solution.