What happens when a buffer's capacity is exceeded by added acid or base?

What happens when a buffer's capacity is exceeded by added acid or base?

• A. The color of pH indicator changes to purple
• B. The solution instantly becomes neutral
• C. The solute dissolves completely
• D. The pH begins to change more with added acid or base

Answer: (D)

When a buffer is overwhelmed by added acid or base, its ability to hold pH steady breaks down. A buffer resists pH changes by using a conjugate pair—typically a weak acid and its conjugate base—to neutralize small amounts of added acid or base. This keeps the pH around a target value. But this buffering has finite capacity. Once that capacity is exceeded, there aren’t enough conjugate base molecules to neutralize incoming H+ or enough weak acid to neutralize OH-. The result is that the hydrogen ion concentration (or hydroxide ion concentration) changes more with each additional amount of acid or base, so the pH shifts more rapidly. In other words, the buffer no longer buffers effectively as more acid or base is added. The other choices don’t reflect what happens when buffering capacity is exceeded: indicator color change depends on the actual pH rather than buffering capacity, neutralization to a specific pH isn’t automatic, and dissolution of a solute isn’t related to the buffer’s resistance to pH change.

When a buffer is overwhelmed by added acid or base, its ability to hold pH steady breaks down. A buffer resists pH changes by using a conjugate pair—typically a weak acid and its conjugate base—to neutralize small amounts of added acid or base. This keeps the pH around a target value. But this buffering has finite capacity. Once that capacity is exceeded, there aren’t enough conjugate base molecules to neutralize incoming H+ or enough weak acid to neutralize OH-. The result is that the hydrogen ion concentration (or hydroxide ion concentration) changes more with each additional amount of acid or base, so the pH shifts more rapidly. In other words, the buffer no longer buffers effectively as more acid or base is added. The other choices don’t reflect what happens when buffering capacity is exceeded: indicator color change depends on the actual pH rather than buffering capacity, neutralization to a specific pH isn’t automatic, and dissolution of a solute isn’t related to the buffer’s resistance to pH change.