Since Lavoisier's knowledge of strong acids was mainly restricted to the oxyacids, which tend to contain central atoms in high oxidation states surrounded by oxygen, such as HNO3 and H2SO4, and since he was not aware of the true composition of the hydrohalic acids, HCl, HBr, and HI, he defined acids in terms of their containing oxygen, which in fact he named from Greek words meaning "acid-former". When the elements chlorine, bromine, and iodine were identified and the absence of oxygen in the hydrohalic acids was established, this definition had to be rejected.
A compound causing an increase in H+ and a decrease in OH- is an acid and one causing the reverse is a base.
The positive ion from a base can form a salt from the negative ion of an acid. For example, two moles of the base sodium hydroxide (NaOH) can combine with one mole of sulfuric acid (H2SO4) to form two moles of water and one mole of sodium sulfate.
The Brønsted-Lowry definition, formulated independently by its two proponents in 1923, revolves around an acid's ability to donate protons (H+) to another compound, called a base, in a chemical reaction.
A base is a proton acceptor. In Brønsted-Lowry acid-base reactions, there is a "competition" between two bases for a proton. so that if X and Y are two species, the equilibrium
occurs. Both HX and HY are Brønsted-Lowry acids; both X- and Y- are Brønsted-Lowry bases. If the reaction runs mostly to the left, then HY is the stronger acid and X- the stronger base; if the reaction runs mostly to the right, then HX is the stronger acid and Y- the stronger base.
Acids and bases in the Brønsted-Lowry system occur in conjugate pairs; in the reaction
Some compounds, like water, can act either as an acid or a base, and are called amphoteric compounds. Stronger acids also typically oxidize metals, forming salts and releasing hydrogen.
See pH for a measure of proton concentration frequently used for measuring acidity and alkalinity using this definition.
This definition is based on a generalization of the earlier Arrhenius definition. If we consider a solvent which can be dissociated into a positive species X and a negative species Y:
a compound causing an increase in X+ (or X2Y+) and a decrease in Y- (or XY2-) is an acid and one causing the reverse is a base. For example in liquid sulfur dioxide (SO2), thionyl[?] compounds (formally supplying SO+2) behave as acids, and sulfites[?] (supplying SO3-2) behave as bases.
In this more general sense, aprotic compounds (those which do not donate protons), can still react with bases, and the terms "acid" and "base" can still be used for reactions in aprotic or non-aqueous environments.
The more general definition offered by Lewis in 1923 (the same year as the Brønsted-Lowry definition) describes the reactivity of an acid in terms of its ability to accept a pair of electrons from a base, defined as an electron-pair donor. In general, an acid reacts with a base by forming a new covalent bond utilizing an empty orbital of the acid to share the extra electron pair of the base.
The most general definition is that of the Russian chemist Usanovich, and can basically be summarized as defining an acid as anything that accepts negative species or donates positive ones, and a base as the reverse. This tends to overlap the concept of redox (oxidation-reduction), and so is not highly favored by chemists.