A substance can often be classified as an acid or a base. This is often done on the basis of a particular kind of reaction, namely the exchange of protons between chemical compounds. However, an extension to this mode of classification was brewed up by the American chemist, Gilbert Newton Lewis; in this mode of classification the reaction is not limited to those occurring in an aqueous solution, thus is no longer limited to solutions in water. According to concept as per Lewis, the crucial things being exchanged are charges[37]. There are several other ways in which a substance may be classified as an acid or a base, as is evident in the history of this concept