When a strong acid is titrated with a strong base using phenolphthalein as an indicator, the color c — Acids and Bases Chemistry Question

STEPS:

1. Identify the Nature of the Titration: The reaction involves a strong acid and a strong base. In such titrations, the net ionic reaction is $H^+(aq) + OH^-(aq) \rightarrow H_2O(l)$, which goes to completion.
2. Analyze the Titration Curve: As seen in standard titration curves, there is a nearly vertical region at the equivalence point. This verticality indicates that the pH changes extremely rapidly with the addition of a very small volume of titrant.
3. Apply the Logarithmic Scale of pH: Because pH is defined as $-\log[H^+]$, a small change in the concentration of hydrogen ions near the equivalence point results in a mathematically large change in pH.
4. Connect pH Change to the Indicator: Phenolphthalein is an organic dye that changes color within a specific pH range (typically 8.2 to 10.0). Near the endpoint, the concentration of $H^+$ or $OH^-$ is so low that one single drop of base provides enough $OH^-$ ions to cause the pH to "jump" across the entire color-change interval of the indicator.
5. Explain the Reversibility: Because the pH is sitting on a steep "cliff" on the titration curve, adding one drop of acid restores a high enough $[H^+]$ to immediately drop the pH back below the indicator's range, switching the color back. Therefore, the abruptness is caused by the solution's pH behavior, not the indicator itself.

WHY_OTHERS_WRONG:

• B: A buffer solution is specifically designed to resist changes in pH. If a buffer existed at the endpoint, the color change would be gradual and slow, requiring many drops to shift the color, which contradicts the "abrupt" observation.
• C: Phenolphthalein and other indicators are typically weak acids or weak bases. If it were a strong proton donor (a strong acid), it would react completely and prematurely with the base, ruining the accuracy of the titration.
• D: The pH of pure water is not resistant to change; it has no buffering capacity. In fact, it is because water cannot resist the addition of $H^+$ or $OH^-$ that the pH jump at the equivalence point is so large.
• E: The "abruptness" is a result of the steepness of the titration curve. Any indicator (such as methyl red or bromothymol blue) would also change color abruptly if its transition range fell on that steep portion of the curve; it is not a unique sensitivity of phenolphthalein.