subject to the theories of Debye, Onsager, and others. An acid and a base react not to produce a salt and a solvent, but to form a new acid and a new base. The concept of neutralization is thus absent. Brønsted–Lowry acid–base behavior is formally independent of any solvent, making it more all-encompassing than the Arrhenius model. The calculation of pH under the Arrhenius model depended on alkalis (bases) dissolving in water (aqueous solution). The Brønsted–Lowry model expanded what could be pH tested using insoluble and soluble solutions (gas, liquid, solid). The general formula for acid–base reactions according to the Brønsted–Lowry definition is: HA + B → BH+ + A− where HA represents the acid, B represents the base, BH+ represents the conjugate acid of B, and A− represents the conjugate base of HA. For example, a Brønsted–Lowry model for the dissociation of hydrochloric acid (HCl) in aqueous solution would be the following: HCl + H2O H3O+ + Cl− The removal of H+ from the HCl produces the chloride ion, Cl−, the conjugate base of the acid. The addition of H+ to the H2O (acting as a base) forms the hydronium ion, H3O+, the conjugate acid of the base. Water is amphoteric—that is, it can act as both an acid and a base. The Brønsted–Lowry model explains this, showing the dissociation of water into low concentrations of hydronium and hydroxide ions: H2O + H2O H3O+ + OH− This equation is demonstrated in the image below: Here, one molecule of water acts as an acid, donating an H+ and forming the conjugate base, OH−, and a second molecule of water acts as a base, accepting the H+ ion and forming the conjugate acid, H3O+. As an example of water acting as an acid, consider an aqueous solution of pyridine, C5H5N. C5H5N + H2O [C5H5NH]+ + OH− In this example, a water molecule is split into a hydrogen ion, which is donated to a pyridine molecule, and a hydroxide ion. In the Brønsted–Lowry model, the solvent does not necessarily have to be water, as is required by the Arrhenius Acid-Base model. For example, consider what happens when acetic acid, CH3COOH, dissolves in liquid ammonia. + + An H+ ion is removed from acetic acid, forming its conjugate base, the acetate ion, CH3COO−. The addition of an H+ ion to an ammonia molecule of the solvent creates its conjugate acid, the ammonium ion,