Here's why:

* Definition of an acid: An acid is a substance that donates a proton (H+) to another substance.

* Dissociation in water: When an acid dissolves in water, it releases H+ ions into the solution.

* Example: Consider hydrochloric acid (HCl). When it dissolves in water, it dissociates into H+ ions and Cl- ions:

HCl (aq) → H+ (aq) + Cl- (aq)

The presence of these H+ ions is what gives acids their characteristic properties, like sour taste and the ability to react with bases.