Kaplan Nursing Entrance Practice Exam

In the presence of an acid, phenolphthalein appears as what color?

1. Pink

2. Clear

3. Blue

4. Green

The correct answer is: Clear

Phenolphthalein is a pH indicator that transitions between different colors at varying pH levels. In acidic environments, its structure is protonated, resulting in a colorless appearance. As the pH rises above around 8.2, phenolphthalein begins to deprotonate, changing to a pink color which indicates a more basic solution. In a strong acid solution, the pH is low, typically below 7, meaning the phenolphthalein remains in its colorless form. This property makes phenolphthalein particularly useful in titrations to determine the endpoint of reactions involving acids and bases, where the change from clear to pink signifies a neutralization reaction has occurred. Understanding this chromatic behavior of phenolphthalein is crucial for interpreting results in laboratory settings, especially in chemistry and biological sciences.