In chemistry, bases are substances that, in aqueous solution, release hydroxide (OH−) ions, are slippery to the touch, can taste bitter if an alkali,^[1] change the color of indicators (e.g., turn red litmus paper blue), react with acids to form salts, promote certain chemical reactions (base catalysis), accept protons from any proton donor or contain completely or partially displaceable OH− ions. Examples of bases are the hydroxides of the alkali metals and the alkaline earth metals (NaOH, Ca(OH)2, etc.—see alkali hydroxide and alkaline earth hydroxide).

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− ions quantitatively. However, it is important to realize that basicity is not the same as alkalinity. Metal oxides, hydroxides, and especially alkoxides are basic, and conjugate bases of weak acids are weak bases.

Bases can be thought of as the chemical opposite of acids. However, some strong acids are able to act as bases.^[2] Bases and acids are seen as opposites because the effect of an acid is to increase the hydronium (H3O+) concentration in water, whereas bases reduce this concentration. A reaction between an acid and a base is called neutralization. In a neutralization reaction, an aqueous solution of a base reacts with an aqueous solution of an acid to produce a solution of water and 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.

For a substance to be classified as an Arrhenius base, it must produce hydroxide ions in an aqueous solution. Arrhenius believed that in order to do so, the base must contain hydroxide in the formula. This makes the Arrhenius model limited, as it cannot explain the basic properties of aqueous solutions of ammonia (NH3) or its organic derivatives (amines).^[3] There are also bases that do not contain a hydroxide ion but nevertheless react with water, resulting in an increase in the concentration of the hydroxide ion.^[4] An example of this is the reaction between ammonia and water to produce ammonium and hydroxide.^[4] In this reaction ammonia is the base because it accepts a proton from the water molecule.^[4] Ammonia and other bases similar to it usually have the ability to form a bond with a proton due to the unshared pair of electrons that they possess.^[4] In the more general Brønsted–Lowry acid–base theory, a base is a substance that can accept hydrogen cations (H+)—otherwise known as protons. In the Lewis model, a base is an electron pair donor.^[5]

General properties of bases include:

The following reaction represents the general reaction between a base (B) and water to produce a conjugate acid (BH+) and a conjugate base (OH−):^[4]

The equilibrium constant, Kb, for this reaction can be found using the following general equation:^[4]

In this equation, the base (B) and the extremely strong base (the conjugate base OH−) compete for the proton.^[7] As a result, bases that react with water have relatively small equilibrium constant values.^[7] The base is weaker when it has a lower equilibrium constant value.^[4]

Bases react with acids to neutralize each other at a fast rate both in water and in alcohol.^[2] When dissolved in water, the strong base sodium hydroxide ionizes into hydroxide and sodium ions:

and similarly, in water the acid hydrogen chloride forms hydronium and chloride ions:

When the two solutions are mixed, the H3O+ and OH− ions combine to form water molecules:

If equal quantities of NaOH and HCl are dissolved, the base and the acid neutralize exactly, leaving only NaCl, effectively table salt, in solution.

Weak bases, such as baking soda or egg white, should be used to neutralize any acid spills. Neutralizing acid spills with strong bases, such as sodium hydroxide or potassium hydroxide, can cause a violent exothermic reaction, and the base itself can cause just as much damage as the original acid spill.

Bases are generally compounds that can neutralize an amount of acids. Both sodium carbonate and ammonia are bases, although neither of these substances contains OH− groups. Both compounds accept H+ when dissolved in protic solvents such as water:

From this, a pH, or acidity, can be calculated for aqueous solutions of bases. Bases also directly act as electron-pair donors themselves:

A base is also defined as a molecule that has the ability to accept an electron pair bond by entering another atom's valence shell through its possession of one electron pair.^[2] There are a limited number of elements that have atoms with the ability to provide a molecule with basic properties.^[2] Carbon can act as a base as well as nitrogen and oxygen. Fluorine and sometimes rare gases possess this ability as well.^[2] This occurs typically in compounds such as butyl lithium, alkoxides, and metal amides such as sodium amide. Bases of carbon, nitrogen and oxygen without resonance stabilization are usually very strong, or superbases, which cannot exist in a water solution due to the acidity of water. Resonance stabilization, however, enables weaker bases such as carboxylates; for example, sodium acetate is a weak base.

A strong base is a basic chemical compound that can remove a proton (H+) from (or deprotonate) a molecule of even a very weak acid (such as water) in an acid-base reaction. Common examples of strong bases include hydroxides of alkali metals and alkaline earth metals, like NaOH and Ca(OH)2, respectively. Due to their low solubility, some bases, such as alkaline earth hydroxides, can be used when the solubility factor is not taken into account.^[8] One advantage of this low solubility is that "many antacids were suspensions of metal hydroxides such as aluminum hydroxide and magnesium hydroxide."^[9] These compounds have low solubility and have the ability to stop an increase in the concentration of the hydroxide ion, preventing the harm of the tissues in the mouth, oesophagus, and stomach.^[9] As the reaction continues and the salts dissolve, the stomach acid reacts with the hydroxide produced by the suspensions.^[9] Strong bases hydrolyze in water almost completely, resulting in the leveling effect."^[2] In this process, the water molecule combines with a strong base, due to the water's amphoteric ability; and, a hydroxide ion is released.^[2] Very strong bases can even deprotonate very weakly acidic C–H groups in the absence of water. Here is a list of several strong bases:

The cations of these strong bases appear in the first and second groups of the periodic table (alkali and earth alkali metals). Tetraalkylated ammonium hydroxides are also strong bases since they dissociate completely in water. Guanidine is a special case of a species that is exceptionally stable when protonated, analogously to the reason that makes Perchloric acid and Sulfuric acid very strong acids. Acids with a pKa of more than about 13 are considered very weak, and their conjugate bases are strong bases.

Group 1 salts of carbanions, amides, and hydrides tend to be even stronger bases due to the extreme weakness of their conjugate acids, which are stable hydrocarbons, amines, and dihydrogen. Usually, these bases are created by adding pure alkali metals such as sodium into the conjugate acid. They are called superbases, and it is impossible to keep them in water solution because they are stronger bases than the hydroxide ion. As such, they deprotonate conjugate acid water. For example, the ethoxide ion (the conjugate base of ethanol) in the presence of water undergoes this reaction.

Examples of superbases are:

When a neutral base forms a bond with a neutral acid, a condition of electric stress occurs.^[2] The acid and the base share the electron pair that formerly only belonged to the base.^[2] As a result, a high dipole moment is created, which can only be destroyed by rearranging the molecules.^[2]

A weak base is one which does not fully ionize in an aqueous solution, or in which protonation is incomplete.

Examples of solid bases include:

Depending on a solid surface's ability to successfully form a conjugate base by absorbing an electrically neutral acid, the basic strength of the surface is determined.^[11] "The number of basic sites per unit surface area of the solid" is used to express how much base is found on a solid base catalyst.^[11] Scientists have developed two methods to measure the amount of basic sites: titration with benzoic acid using indicators and gaseous acid adsorption.^[11] A solid with enough basic strength will absorb an electrically neutral acid indicator and cause the acid indicator's color to change to the color of its conjugate base.^[11] When performing the gaseous acid adsorption method, nitric oxide is used.^[11] The basic sites are then determined using the amount of carbon dioxide than is absorbed.^[11]

Basic substances can be used as insoluble heterogeneous catalysts for chemical reactions. Some examples are metal oxides such as magnesium oxide, calcium oxide, and barium oxide as well as potassium fluoride on alumina and some zeolites. Many transition metals make good catalysts, many of which form basic substances. Basic catalysts have been used for hydrogenations, the migration of double bonds, in the Meerwein-Ponndorf-Verley reduction, the Michael reaction, and many other reactions. Both CaO and BaO can be highly active catalysts if they are treated with high temperature heat.^[11]

The number of ionizable hydroxide (OH-) ions present in one molecule of base is called the acidity of bases.^[12] On the basis of acidity bases can be classified into three types: monoacidic, diacidic and triacidic.

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

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

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

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

When one molecule of base via complete ionization produces three hydroxide ions, the base is said to be triacidic.^[13] Examples of triacidic bases are:

Aluminium hydroxide, ferrous hydroxide, Gold Trihydroxide,^[14]

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