In addition to the operational definitions for acids and bases, a number of theories have been developed to explain the behaviors of these substances. One of earliest and simplest is the Arrhenius Theory. In it, acids and bases are defined in terms of their behavior in WATER.
- Acid: dissolves in water to form H+ ions
- Base: dissolves in water to form OH- ions
Because it is so simple to use, it is easy to fall into the terminology of the Arrhenius model when discussing acid/base behavior informally. But the model suffers from some real deficiencies, not the least of which is the unrealistic aqueous H+ which is the "acid particle". Protons are so small and the unit charge therefore so concentrated that very few--if any--probably exist in water independently.