Acids release hydrogen ions (H+). An Arrhenius acid is a substance which produces H+ ions in aqueous solutions. This concept was proposed by Svante Arrhenius in the 1880s. Hydrogen ions may also be called protons.

H+ ions are also released when a water molecule undergoes autoionization. Autoionization is the process in which a molecule reacts with itself to form ions. In the case of water, two molecules react with each other to form a hydronium ion (H3O+) and a hydroxide ion (OH-). $$2 H_2 O \leftrightharpoons H_3 O^+ + OH$$