Final answer:
H₃PO₄, or phosphoric acid, is a weak acid because it only partially dissociates in water, with most of its molecules staying undissociated.
Step-by-step explanation:
H₃PO₄, also known as phosphoric acid, is classified based on its dissociation properties in water. Acids can be either strong acids that dissociate almost completely in water or weak acids that only partially dissociate in water.
Contrary to strong acids like HCl, a weak acid such as H₃PO₄ does not release all of its hydrogen ions into solution. Most of the phosphoric acid molecules remain undissociated, thereby classifying it as a weak acid.