When phenolphthalein is added to a base, it undergoes a color change from colorless to pink. This is because phenolphthalein is a weak acid that donates a proton (H+) in acidic solutions, resulting in a colorless solution. However, in basic solutions, the hydroxide ions (OH-) from the base react with the protonated phenolphthalein, causing it to lose its proton and form the phenolphthalein anion. This anion is responsible for the pink color observed in basic solutions.

The chemical reaction that takes place can be represented as follows:

HIn (colorless) + OH- (base) → In- (pink) + H2O

Where HIn represents the protonated phenolphthalein and In- represents the phenolphthalein anion.

The color change of phenolphthalein is reversible, meaning that it can change back to colorless if the pH of the solution is lowered again by adding an acid. This property makes phenolphthalein a useful indicator for acid-base titrations, where it is used to signal the equivalence point, or the point at which the acid and base have completely reacted.