DISSOCIATION, in chemistry, is the reso lution of a •substance into other substances, whose molecules have a simpler constitution; but the word is usually applied only to those reversible resolutions in which the secondary products are capable of recombining to form the primitive substance, when the conditions that led to the dissociation are removed or re versed. Dissociation is therefore to be con trasted with for the latter word is used without any implication as to the subsequent behavior of the products into which the primitive substance is resolved.
Dissociation, in the usual sense of the word, is induced directly by the application of heat, and many cases of it have been exhaustively studied. Calcium carbonate may be taken as an example. When this substance is heated in a closed vessel it parts with a portion of its carbon dioxide and becomes reduced to a mix ture of calcium oxide, calcium carbonate and free carbon dioxide gas. The decomposition proceeds only up to a certain limit, however, and if (as in "burning" limestone for the pro duction of quicklime) it is desired to reduce the carbonate of calcium entirely to the form of the oxide, it is necessary to provide for the removal of the carbon dioxide gas. If the operation be carried out in a strong closed vessel which contains nothing but calcium carbonate, calcium oxide and carbon dioxide, the decom position proceeds only until the liberated carbon dioxide attains a definite pressure called the pressure of dissociation, which is always the same for the same temperature, and bears no relation to the quantity of calcium carbonate that is present, provided some quantity of it (no matter how little) still remains. Sir James Hall showed, many years ago, that when calcium carbonate is confined in this way it may even be fused without undergoing decomposition to any considerable extent.
The explanation of these curious facts is, that the carbon dioxide is continually combining with the calcium oxide in certain parts of the vessel with the production of calcium carbonate, while in other parts of the vessel the calcium carbonate is simultaneously dissociating into free calcium oxide and free carbon dioxide.
The recombination proceeds with a velocity that is proportional to the density of the free gas, while the dissociation proceeds with a velocity which, for the purpose of illustration at all events, may be regarded as constant. It follows that at low densities of the gas the dissociation will proceed faster than the recombination, the result being that the density of the free gas will increase. But this will also cause the re combination to proceed faster, and finally, when the density of the gas reaches a certain critical value, the recombination and dissociation will proceed with equal velocity and a state of ap parent equilibrium will be attained. No further visible decomposition of the carbonate will take place, unless the equilibrium is disturbed by the removal of some of the carbon dioxide (in which case the dissociation will again proceed until the same critical density of the free gas is restored), or by a further increase of tem perature (in which case the dissociation will proceed until the density of the gas attains to a new critical value, higher than the first one, and whose value depends upon nothing but the new temperature to which the system has been raised). Reactions such as the one here de scribed thoroughly refute the old doctrines of chemical affinity, which taught that two sub stances either will or will not combine under given circumstances, according as the chemical "affinities" of the constituent elements would be more or less thoroughly satisfied° in the possible new compound than they are in the separate substances that might combine to pro duce that compound. In the case considered above, combination and decomposition go on simultaneously, and the only question is, which of the two will proceed the faster under given conditions. (See EQUILIBRIUM, CHEMICAL).