Here's why:
* Phenolphthalein is an indicator: It changes color depending on the pH of the solution.
* Hydroxide ions (OH-) make solutions basic: Basic solutions have a high pH.
* Phenolphthalein is colorless in acidic solutions: It turns pink in basic solutions.
Therefore, when you add phenolphthalein to a solution of hydroxide (which is basic), it will turn pink.
