A solution that resists changes in pH upon the addition of an acid or base is called a

• A. pH
• B. Base
• C. Solution
• D. Buffer

Answer: (D)

A buffer is a solution that resists changes in pH upon addition of acid or base. It typically contains a weak acid and its conjugate base (or a weak base and its conjugate acid), which work together to neutralize added H+ or OH−. When an acid is added, the conjugate base in the buffer consumes the extra protons to form the weak acid; when a base is added, the weak acid donates protons to neutralize the hydroxide. This keeps the pH around a relatively stable value rather than swinging widely. For example, a mixture of acetic acid and acetate can maintain a steady pH near 4.7. This stability is essential in biological systems and experiments where enzymes and processes require a specific pH range. The other terms describe what a solution is, or what acids/bases do, but they don’t describe a system that resists pH changes the way a buffer does.