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chemical reaction, according to the Arrhenius theory of acids and basesacids and bases,
two related classes of chemicals; the members of each class have a number of common properties when dissolved in a solvent, usually water. Properties
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, in which a water solution of acid is mixed with a water solution of base to form a saltsalt,
chemical compound (other than water) formed by a chemical reaction between an acid and a base (see acids and bases). Characteristics and Classification of Salts

The most familiar salt is sodium chloride, the principal component of common table salt.
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 and water; this reaction is complete only if the resulting solution has neither acidic nor basic properties. Such a solution is called a neutral solution. Complete neutralization can take place when a strong acid, such as hydrochloric acid, HCl, is mixed with a strong base, such as sodium hydroxide, NaOH. Strong acids and strong bases completely break up, or dissociate, into their constituent ions when they dissolve in water. In the case of hydrochloric acid, hydrogen ions, H+, and chloride ions, Cl, are formed. In the case of sodium hydroxide, sodium ions, Na+, and hydroxide ions, OH, are formed. The hydrogen and hydroxide ions readily unite to form water. If the number of hydrogen ions in the hydrochloric acid solution is equal to the number of hydroxide ions in the sodium hydroxide solution, complete neutralization occurs when the two solutions are mixed. The resulting solution contains sodium ions and chloride ions that unite when the water evaporates to form sodium chloride, common table salt. In a neutralization reaction in which either a weak acid or a weak base is used, only partial neutralization occurs. In a neutralization reaction in which both a weak acid and a weak base are used, complete neutralization can occur if the acid and the base are equally weak. The heat produced in the reaction between an acid and a base is called the heat of neutralization. When any strong acid is mixed with any strong base, the heat of neutralization is always about 13,700 calories for each equivalent weightequivalent weight.
The equivalent weight of an element or radical is equal to its atomic weight or formula weight divided by the valence it assumes in compounds. The unit of equivalent weight is the atomic mass unit; the amount of a substance in grams numerically equal to the
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 of acid and base neutralized. See article on pH; titrationtitration
, gradual addition of an acidic solution to a basic solution or vice versa (see acids and bases); titrations are used to determine the concentration of acids or bases in solution.
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(or neutralization reaction), a chemical reaction between a substance with acidic properties and a substance with basic properties that leads to the loss of characteristic properties of both substances. Neutralization is used to establish such properties of acids as the change in color of some soluble indicator dyes (for example, violet litmus to red) under the action of acids, the catalytic effect on some chemical reactions (for example, the inversion of sugars), the solubilizing effect on active metals (for example, magnesium and zinc), carbonates, and other slightly soluble compounds, and the acid taste of aqueous solutions, as well as the loss of all these properties upon reaction with bases. The most typical neutralization reaction in aqueous solutions takes place between hydrated hydrogen ions (also known as hydronium ions) and hydroxide ions, which are found in strong acids and bases, respectively:

H3O+ (or H+ · H2O) + OH- = 2H2O

As a result, the concentration of each ion becomes equal to the concentration characteristic of water itself (about 10 gram-ion per liter at room temperature). In the neutralization of a weak acid by a strong base—for example, neutralization of acetic acid by sodium hydroxide;


the reaction does not proceed to conclusion and is reversible, and the concentration of hydroxide ions in the solution is greater than in pure water (basic reaction of the solution). In the neutralization of a weak base by a strong acid, the solution becomes acidic. Thus, in both the above cases, complete neutralization is not achieved, and the hydrogen ion index (pH) of the solution only approaches 7.

In nonaqueous solutions with prototropic solvents—that is, solvents that are themselves capable of accepting or donating hydrogen ions (protons)—neutralization in the reaction of an acid and a base is attained when the concentration of the solvated hydrogen ions in the solution becomes equal to their concentration in the pure solvent. In nonprototropic solutions of acids and bases, neutralization is achieved upon attainment of the concentration of cations or anions that is characteristic of the pure solvent.

Neutralization reactions are used in the chemical industry and in the treatment of industrial waste, as well as in laboratory practice, especially in chemical analysis.


Shatenshtein, A. I. Teorii kislot i osnovanii. Moscow-Leningrad, 1949.
Day, M. C, and J. Selbin. Teoreticheskaia neorganicheskaia khimiia, 2nd ed. Moscow, 1971. (Translated from English.)
Gyenes, I. Titrovanie ν nevodnykh sredakh. Moscow, 1971. (Translated from English.)



The process of making a solution neutral (pH = 7) by adding a base to an acid solution, or adding an acid to an alkaline (basic) solution. Also known as neutralization reaction.
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