Arrhenius Theory, a postulate on the nature of acids and bases developed by the chemist Svante Arrhenius in the 1890s, was the first real clarification of the chemical nature of acids and bases. His basic theory was that acids contain H+ protons, while bases contain OH-. In other words, an acid is any molecule group R + H+, or in combined format RH. A base is any molecule group R + OH-, or ROH.

The process for deriving this definition was fairly simple and straightforward. He found that reactions of aqueous solutions of hydrogen and aqueous solutions of hydroxide produced liquid water (H2O) and a salt. This process was termed neutralization, the bringing of an acid and base to the neutral pH of 7.0 that characterizes water. Since water was produced from acids and bases, components for water must be contained in the acids and bases. The only way water can be split is into hydrogen and hydroxide, so those must've been the components. Eureeka!

...not quite. It turns out Arrhenius was actually incorrect, although his theory is partially sound. Acids and bases do contain H+ and OH-, but that's not the whole picture. Acids must have a tendency to give up their H+ proton, and bases to receive this proton because of their OH- group, a fact that was outlined in the later Brønsted-Lowry Theory which supplanted Arrhenius' postulate.

So where did Arrhenius go wrong? Well, part of the problem is that aqueous H+ and OH- is not just free hydrogen protons mingling about in water. That is not chemically possible. Rather, what he had was liquid hydronium, H3O+. This happens to be the strongest acid that can exist in water (which isn't saying much, it's extremely weak compared to the rest of the acid spectrum). The hydronium, as an acid under the Brønsted-Lowry Theory, donates its proton to the hydroxide groups, and that is how water is actually produced.

Though his theory was incorrect, Arrhenius made an excellent observation that was extremely useful in the study of physical chemistry, and his achievement should not be devalued.