DISSOCIATION (Lat. dissociatio, from dis soctore, to disrupt, from dis-. away + soeinre, to from socius-. associate). A term applied in chemistry to an important class of de •ompositions. The distinction ween decom position and mhat. is now usually termed dis sociation is diseusseil in the article on DEcoNtro sITION. Another distinction requires tion in the present :ketch—viz. that between chemical dissociation and electrolytie dissocia tion.
Cm:Nue U. DISSOCIATION. Like ordinary chem ical decomposition, chemical dissociation involves the breaking tip of a given chemical substance into parts each of which is capable of independent uxistence. in the case of sal ammoniac (see DE comeosmoN ), the products of dissociation are ammonia gas and hydrochloric acid—two well known chemical compounds. (Idler typical ex amples of chemical dissociation an• presented by the vapor of phosphorus pentoehloride, by the tetroxide of nitrogen, and by ordinary chalk. In the case of phosphorus pentachloride, which is itself colorless. the dissociation is rendered evi dent by the fact that of the two products—viz. phosphors trichloride and free chlorine—the latter is green; this dissociation may therefore be readily demonstrated by heating some phos phorus pentachloride in a glass vessel. Even nn re striking i- the dissociation of the tetroxide f nitrogen, NA)„ which is. at low temperatures and in a pure state, a yellowish, ahnost colorless liquid, but whose vapor breaks up, on the appli cation of beat, into a dark. brownish-red oxide of nitrogen. represented by the formula NO„ Both in the ease of phosphorus pentaehloride and in the ease of nitrogen tetroxide, lowering the temperature causes a recombination of the prod net.: of dissociation and a consequent disappear time of the color. The manner in whirl the dis soc•ation of sal ammoniac may he demonstrated is described in the article on AvocAnao's Butt: (q.v.). It must be observed here that, according to 11. It. Baker (1S93), sal ammoniac does not dissociate at all if very carefully dried, nor do perfectly dry ammonia and hyd•oehhole acid combine to form sal ammoniac. The extent to which a compound may be dissociated is strongly. influenced by two physical tem perature and pressure. Thus. in the ease of nitrogen tetroxide• the a pplieation of a very moderate degree of heat would cause only a fraction of the total amount employed to break up. But as the temperature would be allowed
to rise. the dissociated fraction would increase, and this would be plainly shown by the color of the gas rapidly becoming more and more intense. On the contrary. inereasing pressure would cause the dissociated fraction to diminish, as would be indicated by the gradual disappearance of the color. Careful experiments, carried out at the temperature of about 30° C., have shown that at that temperature nitrogen tetroxide is com pletely dissociated if the pressure is redneed to nothing: under a pressure of about -IfN milli meter- of n•renry, t my a half of the tetroxide is broken up. In the case of a solid substance whose dissociation prod nets include one or More matter. are somewhat different. An vation of temperature causes, just as in the ease of gases, an increase of the dissociated fraction; but the pressure of the gaseous product of sociation cannot be increased. The phenomenon is in this respect perfectly analogous to the evaporation of water. It is well known that for every given tempera t uro the vapor-pressure of water is constant: an attempt to increase that pressure would he unsuccessful and would only cause some of the vapor to comlense. This is precisely what happens in the elassical ease of the dissociation of chalk. Under the influence of heat. chalk breaks up into quicklime and bonic acid. The higher the temperature the less ebalk remain, muli..../wiated and the greater the amount of the carbonic acid produced. But if at a given temperature we should attempt to increase the pressure by diminishing the volume within which t ht. ,•11h,t a 11` COntill(41, the re sult would be that some of the earbonic acid would recombine with the quicklime, and thus the original pressure would s01111 be reestab lished. This fixed pressure is often referred to as the 'dissociation-tension' of chalk, just as the pressure of water-vapor at some given tempera ture is spoken of as the vapor-tension of water at that temperature. Again. just as the vapor tension of water is the same whether the amount of water is large or small, so is the disso•iation tension of a solid substance independent of the amount of the latter experimented upon.