Bronsted-Lowry theory defines acids as proton donors and bases as proton acceptors. This generalizes the definitions of acids and bases beyond just aqueous solutions by considering them as substances that can donate or accept protons in any medium. The Arrhenius definitions of acids producing hydrogen ions and bases producing hydroxide ions in water are limited since they only apply to aqueous solutions and not substances themselves in other mediums like gases. Bronsted and Lowry introduced their proton theory of acids and bases in 1923 to broaden the definitions.