An Arrhenius acid is a substance that dissociates in water to form hydrogen ions (H+). In other words, an acid increases the concentration of H+ ions in an aqueous solution. This protonation of water yields the hydronium ion (H3O+); in modern times, H+ is used as a shorthand for H3O+ because it is now known that a bare proton (H+) does not exist as a free species in aqueous solution.