In Arrhenius acid definition, an acid is a substance that dissociates into hydrogen (H+) and other ions in water. Therefore, the ions present in acids are H+ ions and the other ions that the acid molecule can dissociate into. For example, hydrochloric acid dissociates into H+ and Cl- ions, so the ions present in hydrochloric acid are H+ and Cl-. Similarly, sulfuric acid dissociates into H+ and SO42- ions, so the ions present in sulfuric acid are H+ and SO42-.