thumb|250px|right|[[Soaps are weak bases formed by the reaction of fatty acids with sodium hydroxide or potassium hydroxide.]]

In chemistry, there are three definitions in common use of the word "base": Arrhenius bases, Brønsted bases, and Lewis bases. All definitions agree that bases are substances that react with acids, as originally proposed by G.-F. Rouelle in the mid-18th century.

In 1884, Svante Arrhenius proposed that a base is a substance which dissociates in aqueous solution to form hydroxide ions OH<sup>−</sup>. These ions can react with hydrogen ions (H<sup>+</sup> according to Arrhenius) from the dissociation of acids to form water in an acid–base reaction. A base was therefore a metal hydroxide such as NaOH or Ca(OH)<sub>2</sub>. Such aqueous hydroxide solutions were also described by certain characteristic properties. They are slippery to the touch, can taste bitter, and change the color of pH indicators (e.g., turn red litmus paper blue).

In water, by altering the autoionization equilibrium, bases yield solutions in which the hydrogen ion activity is lower than it is in pure water, i.e., the water has a pH higher than 7.0 at standard conditions. A soluble base is called an alkali if it contains and releases OH<sup>−</sup> ions quantitatively. Metal oxides, hydroxides, and especially alkoxides are basic, and conjugate bases of weak acids are weak bases.

Bases and acids are seen as chemical opposites because the effect of an acid is to increase the hydronium (H<sub>3</sub>O<sup>+</sup>) concentration in water, whereas bases reduce this concentration. A reaction between aqueous solutions of an acid and a base is called neutralization, producing a solution of water and a salt in which the salt separates into its component ions. If the aqueous solution is saturated with a given salt solute, any additional such salt precipitates out of the solution.

In the more general Brønsted–Lowry acid–base theory (1923), a base is a substance that can accept hydrogen cations (H<sup>+</sup>)—otherwise known as protons. This does include aqueous hydroxides since OH<sup>−</sup> does react with H<sup>+</sup> to form water, so that Arrhenius bases are a subset of Brønsted bases. However, there are also other Brønsted bases which accept protons, such as aqueous solutions of ammonia (NH<sub>3</sub>) or its organic derivatives (amines). These bases do not contain a hydroxide ion but nevertheless react with water, resulting in an increase in the concentration of hydroxide ion. Also, some non-aqueous solvents contain Brønsted bases which react with solvated protons. For example, in liquid ammonia, NH<sub>2</sub><sup>−</sup> is the basic ion species which accepts protons from NH<sub>4</sub><sup>+</sup>, the acidic species in this solvent.

G. N. Lewis realized that water, ammonia, and other bases can form a bond with a proton due to the unshared pair of electrons that the bases possess. In the Lewis theory, a base is an electron pair donor which can share a pair of electrons with an electron acceptor which is described as a Lewis acid. The Lewis theory is more general than the Brønsted model because the Lewis acid is not necessarily a proton, but can be another molecule (or ion) with a vacant low-lying orbital which can accept a pair of electrons. One notable example is boron trifluoride (BF<sub>3</sub>).

Some other definitions of both bases and acids have been proposed in the past, but are not commonly used today.

Etymology of the term

The concept of base stems from an older alchemical notion of "the matrix":

Properties

General properties of bases include:

  • Concentrated or strong bases are caustic on organic matter and react violently with acidic substances.
  • Aqueous solutions or molten bases dissociate into ions and conduct electricity.
  • Reactions with indicators: bases turn red litmus paper blue, phenolphthalein pink, keep bromothymol blue in its natural colour of blue, and turn methyl orange-yellow.
  • The pH of a basic solution at standard conditions is greater than seven.
  • Bases are bitter.

Reactions between bases and water

The following reaction represents the general reaction between a base (B) and water to produce a conjugate acid (BH<sup>+</sup>) and a conjugate base (OH<sup>−</sup>):<chem display="block">{B}_{(aq)} + {H2O}_{(l)} <=> {BH+}_{(aq)} + {OH-}_{(aq)}</chem>The equilibrium constant, K<sub>b</sub>, for this reaction can be found using the following general equation:

: <math>K_b = \frac{[BH^+][OH^-]}{[B]}</math>

In this equation, the base (B) and the extremely strong base (the conjugate base OH<sup>−</sup>) compete for the proton. As a result, bases that react with water have relatively small equilibrium constant values. The base is weaker when it has a lower equilibrium constant value.

Neutralization of acids

thumb|[[Ammonia fumes from aqueous ammonium hydroxide (in test tube) reacting with hydrochloric acid (in beaker) to produce ammonium chloride (white smoke).]]

Bases react with acids to neutralize each other at a fast rate both in water and in alcohol. There are a limited number of elements that have atoms with the ability to provide a molecule with basic properties.

:<chem>CH3CH2O- + H2O -> CH3CH2OH + OH-</chem>

Examples of common superbases are:

  • Butyl lithium (n-C<sub>4</sub>H<sub>9</sub>Li)
  • Lithium diisopropylamide (LDA) [(CH<sub>3</sub>)<sub>2</sub>CH]<sub>2</sub>NLi
  • Lithium diethylamide (LDEA)
  • Sodium amide (NaNH<sub>2</sub>)
  • Sodium hydride (NaH)
  • Lithium bis(trimethylsilyl)amide

The strongest superbases are synthesised in only gas phase:

  • Ortho-diethynylbenzene dianion (C<sub>6</sub>H<sub>4</sub>(C<sub>2</sub>)<sub>2</sub>)<sup>2−</sup> (the strongest superbase ever synthesized)
  • Meta-diethynylbenzene dianion (C<sub>6</sub>H<sub>4</sub>(C<sub>2</sub>)<sub>2</sub>)<sup>2−</sup> (second strongest superbase)
  • Para-diethynylbenzene dianion (C<sub>6</sub>H<sub>4</sub>(C<sub>2</sub>)<sub>2</sub>)<sup>2−</sup> (third strongest superbase)
  • Lithium monoxide anion (LiO<sup>−</sup>) was considered the strongest superbase before diethynylbenzene dianions were created.

Weak bases

A weak base is one which does not fully ionize in an aqueous solution, or in which protonation is incomplete. For example, ammonia accepts a proton from water according to the equation

:NH<sub>3</sub>(aq) + H<sub>2</sub>O(l) &rarr; NH(aq) + OH<sup>−</sup>(aq)

The equilibrium constant for this reaction at 25&nbsp;°C is 1.8 &times; 10<sup>−5</sup>, such that the extent of reaction or degree of ionization is quite small.

Lewis bases

A Lewis base or electron-pair donor is a molecule with one or more high-energy lone pairs of electrons which can be shared with a low-energy vacant orbital in an acceptor molecule to form an adduct. In addition to H<sup>+</sup>, possible electron-pair acceptors (Lewis acids) include neutral molecules such as BF<sub>3</sub> and high oxidation state metal ions such as Ag<sup>2+</sup>, Fe<sup>3+</sup> and Mn<sup>7+</sup>. Adducts involving metal ions are usually described as coordination complexes.

According to the original formulation of Lewis, when a neutral base forms a bond with a neutral acid, a condition of electric stress occurs.

  • Mounted bases: LiCO<sub>3</sub> on silica; NR<sub>3</sub>, NH<sub>3</sub>, KNH<sub>2</sub> on alumina; NaOH, KOH mounted on silica on alumina

Uses of bases

  • Sodium hydroxide is used in the manufacture of soap, paper, and the synthetic fiber rayon.
  • Calcium hydroxide (slaked lime) is used in the manufacture of bleaching powder.
  • Calcium hydroxide is also used to clean the sulfur dioxide, which is caused by the exhaust, that is found in power plants and factories.
  • Magnesium hydroxide is used as an 'antacid' to neutralize excess acid in the stomach and cure indigestion.
  • Sodium carbonate is used as washing soda and for softening hard water.
  • Sodium bicarbonate (or sodium hydrogen carbonate) is used as baking soda in cooking food, for making baking powders, as an antacid to cure indigestion and in soda acid fire extinguisher.
  • Ammonium hydroxide is used to remove grease stains from clothes

Monoprotic and polyprotic bases

Bases with only one ionizable hydroxide (OH<sup>−</sup>) ion per formula unit are called monoprotic since they can accept one proton (H<sup>+</sup>). Bases with more than one OH- per formula unit are polyprotic.

The number of ionizable hydroxide (OH<sup>−</sup>) ions present in one formula unit of a base is also called the acidity of the base. On the basis of acidity bases can be classified into three types: monoacidic, diacidic and triacidic.

Monoacidic bases

thumb|[[Sodium hydroxide]]

When one molecule of a base via complete ionization produces one hydroxide ion, the base is said to be a monoacidic or monoprotic base. Examples of monoacidic bases are:

Sodium hydroxide, potassium hydroxide, silver hydroxide, ammonium hydroxide, etc.

Diacidic bases

When one molecule of base via complete ionization produces two hydroxide ions, the base is said to be diacidic or diprotic. Examples of diacidic bases are:

thumb|[[Barium hydroxide]]

Barium hydroxide, magnesium hydroxide, calcium hydroxide, zinc hydroxide, iron(II) hydroxide, tin(II) hydroxide, lead(II) hydroxide, copper(II) hydroxide, etc.

Triacidic bases

When one molecule of base via complete ionization produces three hydroxide ions, the base is said to be triacidic or triprotic. Examples of triacidic bases are:

Aluminium hydroxide, ferric hydroxide, gold(III) hydroxide.