Page:The New International Encyclopædia 1st ed. v. 06.djvu/351

* DISSOCIATION. 301 DISSOCIATION. other ion, canyiiij; an equal cliarge of the oppo- site form of elect ric'ily. Free ehlorine gas would, however, form from the chlorine ions if the latter lost their electric charges; which is precisely what happens wlicn the solution is suhjci'ted to a process of electrolysis. When the ehxtrodes of a galvanic battery are ininier.scd iu the solution, the negative chlorine ions are attracted toward the positive electrode, by which their electric charge is neutralized. The chlorine ions then become simply chlorine atoms, and these com- bine in pairs, forming molecules of free chlorine gas. At the same time the positive ions are attracted and transformed into a free chemical substance, at the negative electrode; for neither can electro-positive nor electro-negative ions ex- ist alone. A current of electricity is then said to pass through the solution, and the latter is said to be electrolyzed. Xow, if this explanation of the mechanism of electrolysis be correct, it is evident that the electrical conductivity of a solu- tion must he proportional to the number of free ions in it ; and lience the numbers of free ions in different solutions may be ascertained by measur- ing the conductivities of the solutions. It is equall.v evident that, if the explanation is cor- rect, the total number of purlicles (ions as well as molecules) in a solution, say. of potassium chloride is greater than what it would be if that salt was not dissociated. Comparing a solution of an electrolyte (i.e. an acid, base, or salt) with a solution of an equivalent quantity of any non-electrolyte, the former solution must contain a larger total number of particles than the lat- ter; because in the latter the number of particles is not increased by dissociation. These considerations have been applied to ex- plain the abnormal behavior of solutions of acids, bases, and salts. A'hen dissolved in water, these substances have been known to aflfect its physical properties much more than equivalent amounts of substances of any other class. Thus, dissolv- ing a given amount of common salt causes the boiling temperature of water to become much higher than dissolving an equivalent amount of sugar (a non-electrolyte) ; and the same may be said of the lowering of the freezing-point of water and of certain other changes in its physical properties. In the case of a non-electrolyte, the elevation of the boiling-point, as well as the de- pression of the freezing point, is found to be pro- portional to the amount of substance dissolved, and can therefore be readily calculated. In the ease of electrolytes, and only of these, that pro- portionality is not found to exist; and before the invention by Arrhenius of the theory of elec- trolytic dissociation, the exact change in physical properties of the solvent produced liy these sub- stances could not be calculated. The theory of dissociation attacRs this problem in the follow- ing manner: It starts with the principle that the changes are proportional to the number of par- ticles of the dissolved substance; combines this principle with the idea that the nimiber of free ions in a solution is sliown by the electrical con- ductivity; calculates the total relative number of particles from the relative number of free ions present; and then calculates the elevation of the boiling-point and the depression of the freezing- point from the total number of particles. The figures thus obtained by theoretical calculation are found to be in remarkably close agreement with the results of actual observation, and this indicates more strongly than any other fact could the great probability of the correctness of the theory of dissociation. Electrolytic dissociation is usually incomplete. That is to say, of the total number of molecules in solution, only some are usually dissociated into ions, the rest remaining undissociated. The dissociated fraction, representing the 'degree of dissociation,' depends on two factors: the con- centration of the solution and the temperature. When the concentration is diminished by adding pure water to the solution, the degree of disso- ciation becomes greater; by adding a very large amount of water to the solution, the dissocia- tion may in many eases be rendered complete, i.e. no niolcrulcs may be left undissociated. As to tlie inllucnce of temperature, no clear causal relation has as yet been established: empirically it is known that a rise of temperature causes in some cases an increase, in other cases a decrease, of the degree of dissociation. Comparing the two forms of dissociation, chemical and electrolytic, we find that they resemble each other with regard to concentration, but differ with regard to tem- perature. Increasing the pressure of a gas causes its volume to diminish, and hence its con- centration, i.e. the amount of matter in unit volume, to increase. A similar rule holds good in the case of electrolytic dissociation in soluti(m. On the other hand, a rise of temperature invaria- bly causes more molecules to dissociate chemic- ally, but in many case=- has the reverse effect on substances dissociated electrolrtically. The the- ory of electrolytic dissociation performs in a re- markable degree the function of a truly scien- tific theory, viz. that of bringing out a connection between phenomena that are seemingly very dif- ferent in their nature. Who would imagine, for instance, that the boiling or freezing temperature of a solution of common salt has any connection with its electrical conductivity? Arrhenitis's the- ory establishes an exact mathematical relation. The theory draws for us a picture of things that lie as far Ijcyond the power of our senses as the remotest of the unseen worlds in s]iace; yet the more proofs are adduced of the correctness of the theory, the more probable it becomes that the picture drawn by it is actually true to reality. To the practical scientist, however, this is of sec- ondary importance. JIuch nun-e important is the fact that the ionic theory furnishes indisputably valuable icorkinfi principles. It can be and has been criticised; and scientific criticism is always useful. Xevertheless. many able chemists are de- voting their energ;t' to develop it and apply it to all possible cases. Let the theory be abandoned as inadequate to-morrow; to-day it leads us to the discovery of new relations, and it has corre- lated (i.e. explained) a variety of facts that were unintelligible yesterday. It has already ex- plained many of the facts of organic and analyti- cal chemistry; it has explained the behavior of the different indicators used in alkalimetry and acidimetn' ; and it is being more and more ex- tensively applied to prolilems of physiology and bacteriology. Tlius, the researches of Louis Kah- Icnberg have shown that the poisonous action of acids, bases, and salts depends on the degree of their dissociation in solutions. Similar results were obtained by Paul and Krfinig, who investi- gated the action of aci<ls and bases, and of the salts of mercury, gold, and silver, on the spores of the anthrax bacillus and on the vegetative