Chemists use several different definitions of acids, as follows.
A substance that contains hydrogen
and dissociates in water to produce positive
hydrogen ions, i.e., protons. Such acids
tend to be corrosive substances with a sharp taste,
which turn litmus red and give color
changes with other indicators. They
are referred to as protonic acids and are classified into strong acids,
which are almost completely dissociated in water (e.g., sulfuric
acid, nitric acid, and hydrochloric
acid), and weak acids, which are only partially dissociated (e.g.,
acetic acid and hydrogen
sulfide). The strength of an acid depends on the extent to to which
it dissociates, and is measured by its dissociation constant. Acids
have pH values ranging from 1 (for a strong
acid) to just under 7 (for a very weak acid). Compare with alkali.
Lowry-Brønsted theory (1923)
In this, the definition is extended to one in which an acid is a proton
donor (a Brønsted acid), and a base
is a proton acceptor (a Brønsted base). An important feature
of the Lowry-Brønsted concept is that when an acid gives up a
proton, a conjugate base is formed that is capable of accepting a proton.
Similarly, every base produces its conjugate acid as a result of accepting
a proton. For example, the acetate ion
is the conjugate base of acetic acid, and the ammonium ion is the conjugate
acid of ammonia. As the acid of a conjugate
acid/base pair becomes weaker, its conjugate base becomes stronger and
A further extension of the idea of acids and bases. A Lewis acid is
a compound or atom that can accept a pair of electrons
and a Lewis base is one that can donate an electron pair. This definition
encompasses "traditional" acid-base reactions, but it also includes
reactions that do not involve ions, e.g. H3N: BCl3
H3NBCl3 in which NH3 is the base (donor)
and BCl3 the acid (acceptor).
Occurrence and uses of acids
Many chemical reactions are speeded up in acid solution, giving rise to
important industrial applications (acid-base catalysis).
Mineral acids, including sulfuric acid, nitric acid, and hydrochloric acid,
find widespread use in industry.
Organic acids, which occur widely in nature, tend to be weaker. Carboxylic
acids (including acetic acid and oxalic
acid) contain the acidic group –COOH; aromatic
systems with attached hydroxyl group (phenols)
are often also acidic. Amino acids, constitutive
of proteins, are essential components of
all living systems on Earth.