When acids are dissolved in water, they donate H+ ions (hydronium ions). The strength of an acid depends on the extent of ionization. The more readily the acid dissociates, the more H+ ions are available. Strong acids, such as hydrochloric acid (HCl), completely ionize in water, producing a large concentration of H+ ions. Weak acids, such as acetic acid (CH3COOH), partially dissociate, resulting in a lower concentration of H+ ions.