The use of polar solvents and the presence of weak bonds involving hydrogen introduces an interesting and complex situation for describing matter whereby protons can be removed or added to molecules or allowed to "flow freely" in the solvent (e.g. water). The study of these phenomena requires the introduction of the acid concept, and also, the equilibration concept. Rarely are the two not intertwined.
Thumbnail: The picture above illustrates the electron density of hydronium. The red area represents oxygen; this is the area where the electrostatic potential is the highest and the electrons are most dense.