Phenolphthalein is a chemical compound that is often used as an indicator in acid-base titrations. It is colorless in acidic solutions and turns pink in basic solutions. The chemical reaction that takes place between phenolphthalein and NaOH is:

Phenolphthalein (HIn) + NaOH → NaIn + H2O

In this reaction, the hydroxide ions (OH-) from NaOH react with the phenolphthalein molecule (HIn) to form the phenolphthalein anion (In-) and water (H2O). The phenolphthalein anion is pink in color, which is why the solution turns pink when NaOH is added.

This reaction is reversible, and the equilibrium constant for the reaction can be expressed as:

K = [In-]/[HIn][OH-]

The equilibrium constant for this reaction is very large, which means that the reaction proceeds almost completely to the right. This means that almost all of the phenolphthalein is converted to the phenolphthalein anion when NaOH is added.