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in chemistry, separation of a substance into atoms or ions. Thermal dissociation occurs at high temperatures. For example, hydrogen molecules (H2) dissociate into atoms (H) at very high temperatures; at 5,000°K; about 95% of the molecules in a sample of hydrogen are dissociated into atoms. Electrolytic dissociation occurs when an electrolyteelectrolyte
, electrical conductor in which current is carried by ions rather than by free electrons (as in a metal). Electrolytes include water solutions of acids, bases, or salts; certain pure liquids; and molten salts.
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 is dissolved in a polar solventsolvent,
constituent of a solution that acts as a dissolving agent. In solutions of solids or gases in a liquid, the liquid is the solvent. In all other solutions (i.e., liquids in liquids or solids in solids) the constituent that is present in larger quantity is considered the
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. For example, when hydrogen chloride, HCl, is dissolved in water to form hydrochloric acid, most of its molecules dissociate into hydrogen 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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 (H+) and chloride ions (Cl). Some pure substances spontaneously dissociate. For example, in pure water some of the molecules dissociate to form hydrogen ions and hydroxyl ions. Dissociation is generally reversible; when the atoms or ions of the dissociated substance are returned to the original conditions, they recombine in the original form of the substance. The dissociation constant is a measure of the extent of dissociation. It is represented by the symbol K. In the simplest case, if a substance AB dissociates into two parts A and B and the concentrations of AB, A, and B are represented by [AB], [A], and [B], then K=[A]×[B]/[AB]. The dissociation constant is measured at equilibrium, and its value is usually affected by changes in temperature.
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The following article is from The Great Soviet Encyclopedia (1979). It might be outdated or ideologically biased.



the process by which molecules break up into a number of simpler particles—molecules, radicals, atoms, or ions. Three types of dissociation are generally distinguished: thermal dissociation, which takes place at elevated temperature (for example, N2O4 ⇄ 2NO2); electrolytic dissociation, which occurs when electrolytes are dissolved (the splitting of the molecules of electrolytes into ions, for example, KOH ⇄ K+ + OH-); and photochemical dissociation, which is observed under the action of light (for example, Cl2 + hγ → 2Cl, where hy is a quantum of light). A quantitative characteristic of dissociation is the degree of dissociation, which is the ratio of the number of molecules that have broken up to the total number of molecules.

The Great Soviet Encyclopedia, 3rd Edition (1970-1979). © 2010 The Gale Group, Inc. All rights reserved.


Independent, uncoordinated functioning of the atria and ventricles.
The appearance of a novel colony type on solid media after one or more subcultures of the microorganism in liquid media.
(physical chemistry)
Separation of a molecule into two or more fragments (atoms, ions, radicals) by collision with a second body or by the absorption of electromagnetic radiation.
The segregation of ideas from their affects or feelings, resulting in independent functioning of these components of a person's mental processes.
McGraw-Hill Dictionary of Scientific & Technical Terms, 6E, Copyright © 2003 by The McGraw-Hill Companies, Inc.
References in periodicals archive ?
The patient was involved in an ATV accident and sustained a type C3, M3 (AO Spine classification) spinopelvic dissociation in addition to anterior pelvic ring disruption with 5 cm of pubic symphysis diastasis.
Schildhauer and colleagues showed that surgical timing did not correlate with clinical outcomes in patients with cauda equina secondary to spinopelvic dissociation [8].
Since in the literature dissociations between different explicit tests (e.g., between recall and recognition, see MacLeod & Kampe, 1996) are also known, the notion here is that explanatory principles, similar to the ones employed to understand those dissociations, can also explain the dissociations between indirect and direct tests.
The systems approach could simply increase the number of postulated systems in order to accommodate the newly obtained dissociations. In fact, that has been done (see Schacter, 1990; Schacter et al., 1993; 1994; Squire, 1994).
When an incompatibility between the two arises, however, appearances that do not conform to our concept of reality are labeled erroneous through the process of dissociation. Perelman and Olbrechts-Tyteca write, "[T]he effect of determining reality is to dissociate those appearances that are deceptive from those that correspond to reality" (p.
They majority of scholars who have studied dissociation have noted its usefulness for defining categories by removing inconsistencies, essentially making those categories narrower by separating out concepts that are not in accordance with the preferred term, term II.
Three variables highly relevant to the debate on the hypnotic state were included: absorption, dissociation, and altered state.
Decisively, the psi scores failed to relate to dissociation as measured by the DES, but they did show a moderately significant relationship to absorption.
The longer we have been occupied with these phenomena the more we have become convinced that the splitting of consciousness which is so striking in the well-known classical cases under the form of 'double conscience' is present to a rudimentary degree in every hysteria, and that a tendency to such a dissociation, and with it the emergence of abnormal states of consciousness (which we shall bring together under the term 'hypnoid'), is the basic phenomenon of this neurosis.
Although Freud began with dissociation as the foundation of his psychoanalytic theory, he later discarded this concept and instead proposed "repression" as a psychological defence mechanism involved in the formation of neurosis.
Nevertheless, research provides strong evidence that assumptions of a dissociative continuum may not be valid for pathological dissociation but for nonpathological dissociative tendency (8, 16).

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