Phenolphthalein is a weak acid that is colourless in its acidic form. When it is added to an acid, it undergoes a chemical reaction called protonation, in which the hydrogen ion (H+) from the acid binds to the phenolphthalein molecule.

This causes the phenolphthalein to become protonated and take on a positive charge. The protonated phenolphthalein molecule then undergoes a second chemical reaction called tautomerization, in which it rearranges its molecular structure to form a new compound called a quinone.

The quinone is responsible for the pink colour that is associated with phenolphthalein in acidic solutions. Therefore, when phenolphthalein is added to an acid, it turns pink.