Which statement best describes a buffer?

• A. A substance that increases pH to a higher value
• B. A substance that fully dissociates in water
• C. A solution that minimizes changes in pH when acids or bases are added
• D. A compound that acts as a solvent

Answer: (C)

A buffer resists changes in pH when acids or bases are added. It accomplishes this by containing a weak acid and its conjugate base (or a weak base and its conjugate acid) that can react with added H+ or OH−. If acid is added, the conjugate base neutralizes some of the added protons, forming the weak acid and keeping the pH from dropping much. If base is added, the weak acid donates protons to neutralize some hydroxide, reforming the conjugate base and limiting the pH rise. This balancing act keeps the pH around a target value (near the acid’s pKa) and depends on the amounts of the two components. It is not about increasing pH by itself, it isn’t defined by complete dissociation, and it isn’t a solvent. That’s why this description best captures what a buffer does.