Page:The New International Encyclopædia 1st ed. v. 06.djvu/350

* DISSENTERS. 300 DISSOCIATION. TioN; Nonconformists; also Estabusiiments, Ecclesiastical. DISSEP'IMENT. A partition in plants such as Uivide^ an ovary into fonipartments, equiva- lent to a trania in iiynienoniycetous fungi. DISSOCIATION (Lat. dissocintio, from dis- iocuuc. to Ji=.iupt, from dis-, away + sociare, to a^soc•iate, from socius, associate). A temi applied in chemistry to an important class ot Uc- cotiipositions. The distinction botxvecn decom- position and what is n.nv usually termed dis- sociation is discussed in the article on Decompo- sition. Another distinction requires cxphina- tion in the present sketch— viz. that between chemical dissociation and electrolytic dissocia- tion. CliEMlcvL Dissociation, l-ike ordinary chem- ical decomposition, chemical dissociation involves the breaking up of a given cliemical substance into parts each of which is capable ot independent existence. In the case of sal ammoniac (see De- COMI'OSITION ), the products of dissociation are ammonia gas and hydrochloric acid— two well- known chemical compounds. Other typical ex- amples of chemical dissociation are presented by the vapor ov phosphorus pentachlonde, by the tetroxide of nitrogen, and by ordinary chalk, in the case of phosphorus pentachlonde, which is itself colorless, the dissociation is rendered evi- dent by the fact that of the two products— viz. phospliorus trichloride and free chlorine— the latter is "rceii : this dissociation may therefore be readily demonstrated by heating some phos- phorus pentachloride in a glass vessel. Kvcn more striking is the dissociation of the tetroxide of nitrogen. X,0„ which is, at low temperatures and in a pure state, a yellowish, almost colorless liquid, but whose vapor breaks up, on the appli- cation of heat, into a dark, biownishvcd oxulo of nitrosen. represented by the fornnila >U;. Both in the case of phosphorus pentachlonde and in the case of nitrogen tetroxide. lowering the temperature causes a recombination of the prod- ucts of dissociation and a consequent disappear- ance of the color. The manner in which the dis- sociation of sal ammoniac may be demonstrated is described in the article on Avogadko s Kule (q v ) It must be observed here that, according to H B Baker (1893), sal ammoniac does not dissociate at all if very carefully dried, nor do perfectly dry ammonia and hydrochloric acid combine to form sal ammoniac. The extent to which a compound may be dissociated is strongly • inlluenced by two physical factors— viz. tem- perature and pressure. Thus, in the case of nitrogen tetroxide, the application 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 rapidlv becoming more and more intense. On the contrary, increasing 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 50° C. have shown that at that temperature nitrogen tetroxide is com- pletely dissociated if the pressure is reduced to nothing: under a pressure of about 408 milli- meters of mercury. . nly a half of the tetroxide is broken up. In the case of a solid substance whose dissociation products include one or more gases, matters are somewhat dilTerent. An ele- vation of temperature causes, just as in the case of gases, an increase of the dissociated fraction; but'' the pressure of the gaseous product of dis- sociation cannot be increased. The phenomenon is in this respect perfectly analogous to the evaporation of water. It is well known that for everv given tcmperulure the vapor-pressure o^ water is constant; an attempt to increase that pressure would be unsuccessful and would only cause some of the vapor to condense. This is precisely what happens in the classical case of the dissociation of chalk. I'nder the inlluence of heat, chalk breaks up into iiuicklimc and car- bonic acid. The higher the temperature the less chalk remains undissociatcd 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 the substances are confined, the re- sult would be that some of the carbonic acid would recombinc with the quicklime, and thus the original pressure would soon 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 s])okcn of as the vapor-tension of water at that temperature. Again, just as the vapor- tension of water is the same whether the amount nf water is large or small, so is the dissociation- tension of a solid substance independent of the amount of the latter experimented iM'""- Electrolytic Dissociation. Quite dillercnt from the phenomenon of cliemical dissociation is electrolytic dissociation: and it must be borne in mind that while the former is a fact, the latter is theoretically inferred to be the cause of the exceptional behavior of many solutions, but cannot be gotten at directly. ' Xevertheless the theory of electrolytic dissociation is now alinost •Generally accepted by chemists, and careful in- yesti^ation brings forward, month after month, new proofs of the extreme probability of its cor- rectness and usefulness. According to this the- ory acids, bases, and salts break up. when dis- solved in water, into atoms or atomic groups, some of which are charged with positive, others with negative electricity. Thus, hydrochloric acid is supposed to dissociate according to the following equation: Ha = H-+-Cl; caustic potash dissociates as follows: + KOH = K + OH; the salt called potassium chloride, as follows: KCl = K + CT. These equations must not. however, be taken to mean that, for example, a solution of potassium cliloridc contains chlorine gas and the metal po- tassium in the free state. Indeed, comparing a solution of potassium chloride with one of chlo- rine gas. it may be seen that while some free chlorine is c.mstantlv escaping from the latter, no free chlorine can be detected in the former; a solution of potassium chloride exhibits neither the color, nor the odor, nor the bleaching powder of free chlorine. The symbol CI denotes what is called an electronecative chlorine 'ion, and the existence of an 'ion* is not an independent exist- ence but involves necessarily the existence of an-