The essence of Brønsted–Lowry theory is that an acid only exists as such in relation to a base, and vice versa. Water is amphoteric as it can act as an acid or as a base. In the image shown at the right one molecule of acts as a base and gains to become while the other acts as an acid and loses to become . Another example is furnished by substances like aluminium hydroxide, . \overset{(acid)}{Al(OH)3}{} + OH- <=> Al(OH)4^- 3H+{} + \overset{(base)}{Al(OH)3} <=> 3H2O{} + Al_{(aq)}^3+ Non-aqueous solutions The hydrogen ion, or hydronium ion, is a Brønsted–Lowry acid in aqueous solutions, and the hydroxide ion is a base, by virtue of the self-dissociation reaction H2O + H2O <=> H3O+ + OH- An analogous reaction occurs in liquid ammonia NH3 + NH3 <=> NH4+ + NH2- Thus, the ammonium ion, , plays the same role in liquid ammonia as does the hydronium ion in water and the amide ion, , is analogous to the hydroxide ion. Ammonium salts behave as acids, and amides behave as bases. Some non-aqueous solvents can behave as bases, that is, proton acceptors, in relation to Brønsted–Lowry acids. HA + S <=> A- + SH+ where S stands for a solvent molecule. The most important such solvents are dimethylsulfoxide, DMSO, and acetonitrile, , as these solvents have been widely used to measure the acid dissociation constants of organic molecules. Because DMSO is a stronger proton acceptor than the acid becomes a stronger acid in this solvent than in water. Indeed, many molecules behave as acids in non-aqueous solution that do not do so in aqueous solution. An extreme case occurs with carbon acids, where a proton is extracted from a bond. Some non-aqueous solvents can behave as acids. An acidic solvent will increase basicity of substances dissolved in it. For example, the compound is known as acetic acid because of its acidic behaviour in water. However it behaves as a base in liquid hydrogen chloride, a much more acidic solvent. HCl + CH3COOH <=> Cl- + CH3C(OH)2+ Comparison with Lewis acid–base theory In the same year that Brønsted and Lowry published their theory, G. N. Lewis proposed an alternative theory of acid–base reactions. The Lewis theory is based on electronic structure. A Lewis base is defined as a compound that can donate an electron pair to a Lewis acid, a compound that can accept an electron