What term describes a substance that is so strongly acidic or alkaline that it resists change to another pH?

• A. Catalyst
• B. Buffer
• C. Surfactant
• D. Emulsifier

Answer: (B)

The idea being tested is how substances resist changes in pH. A buffer is a solution that helps keep pH from shifting when small amounts of acid or base are added. It does this with a combination of a weak acid and its conjugate base (or a weak base and its conjugate acid). When acid is added, the conjugate base neutralizes some of the extra hydrogen ions, forming the weak acid and limiting a pH drop. When base is added, the weak acid donates hydrogen ions to neutralize some of the hydroxide, limiting a pH rise. The buffering effect is strongest near the pKa of the acid involved, and the capacity depends on the concentrations of the buffer components. Other terms listed don’t serve this role: a catalyst speeds up reactions, a surfactant lowers surface tension, and an emulsifier helps mix immiscible liquids.