Final answer:
Calcium hydroxide turns pink after phenolphthalein is added due to an acid-base neutralization reaction, as phenolphthalein changes color in a basic solution.
Step-by-step explanation:
The reaction that causes calcium hydroxide to turn pink after phenolphthalein is added is an acid-base neutralization reaction. Phenolphthalein is a pH indicator that turns pink in a basic solution. Since calcium hydroxide (Ca(OH)2) is a base, it increases the pH of the solution. When a few drops of phenolphthalein are added to calcium hydroxide, the solution turns pink because phenolphthalein changes color in the pH range of 8.2 to 10, indicating a basic solution.