common-ion effect

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common-ion effect,

decrease in solubility of an ionic 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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, i.e., one that dissociates in solution into its ionsion,
atom or group of atoms having a net electric charge. Positive and Negative Electric Charges

A neutral atom or group of atoms becomes an ion by gaining or losing one or more electrons or protons.
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, caused by the presence in solutionsolution,
in chemistry, homogeneous mixture of two or more substances. The dissolving medium is called the solvent, and the dissolved material is called the solute. A solution is distinct from a colloid or a suspension.
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 of another solute that contains one of the same ions as the salt. The common-ion effect is an example of chemical equilibriumchemical equilibrium,
state of balance in which two opposing reversible chemical reactions proceed at constant equal rates with no net change in the system. For example, when hydrogen gas, H2, and iodine gas, I2
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. For example, silver chloride, AgCl, is a slightly soluble salt that in solution dissociates into the ions Ag+ and Cl, the equilibrium state being represented by the equation AgClsolid ⇋Ag++Cl. According to Le Châtelier's principleLe Châtelier's principle,
chemical principle that states that if a system in equilibrium is disturbed by changes in determining factors, such as temperature, pressure, and concentration of components, the system will tend to shift its equilibrium position so as to
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, when a stress is placed on a system in equilibrium, the system responds by tending to reduce that stress. In the system taken as an example, if another solute containing one of those ions is added, e.g., sodium chloride, NaCl, which supplies Cl ions, the solubility equilibrium of the solution will be shifted to remove more Cl from the solution, so that at the new equilibrium point there will be fewer Ag+ ions in solution and more AgCl precipitated out as a solid.
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common-ion effect

[¦käm·ən ¦ī‚än i′fekt]
The lowering of the degree of ionization of a compound when another ionizable compound is added to a solution; the compound added has a common ion with the other compound.
McGraw-Hill Dictionary of Scientific & Technical Terms, 6E, Copyright © 2003 by The McGraw-Hill Companies, Inc.