Which term is a weak acid or base that tends to resist pH change?

• A. Compound
• B. Organic Compound
• C. Solvent
• D. Buffer

Answer: (D)

Buffers resist changes in pH by containing a weak acid and its conjugate base (or a weak base and its conjugate acid). When more H+ is added, the conjugate base captures it to form the weak acid, dampening a drop in pH. When OH− is added, the weak acid donates an H+ to form water and the conjugate base, dampening a rise in pH. This equilibrium keeps the pH relatively stable over a range of added acids or bases. A common example is acetic acid with its conjugate base acetate, or ammonia with ammonium. A general compound or an organic compound is too broad, and a solvent isn’t defined by this buffering action. The defining term for this resisting of pH change is a buffer.