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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.



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.


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.
References in periodicals archive ?
Reliability of electromechanical dissociation in the diagnosis of left ventricular free wall rupture in acute myocardial infarction.
After six minutes of exercise, she collapsed and had a cardiac arrest with electromechanical dissociation (EMD).

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