A substance added to water containing phenolphthalein, causing the solution to turn pink. Which substance would produce this result?

A substance added to water containing phenolphthalein, causing the solution to turn pink. Which substance would produce this result?

• A. KOH
• B. HC2H3O2
• C. H2CO3
• D. CH3OH

Answer: (A)

Phenolphthalein serves as a pH indicator that is colorless in acidic or neutral solutions and turns pink in basic solutions (typically when the pH is above about 8.2). Adding potassium hydroxide to water creates a strongly basic solution, pushing the pH into the range where phenolphthalein appears pink. The other substances are acids or nearly neutral: an acetic acid solution and a carbonic acid solution stay acidic and keep the indicator colorless, while methanol is essentially neutral in water and does not raise the pH enough to trigger the color change.

Phenolphthalein serves as a pH indicator that is colorless in acidic or neutral solutions and turns pink in basic solutions (typically when the pH is above about 8.2). Adding potassium hydroxide to water creates a strongly basic solution, pushing the pH into the range where phenolphthalein appears pink. The other substances are acids or nearly neutral: an acetic acid solution and a carbonic acid solution stay acidic and keep the indicator colorless, while methanol is essentially neutral in water and does not raise the pH enough to trigger the color change.