The quantitative study of the phenomena of dissociation is beset with difficulties, for the extent of the dissociation cannot be ascertained with satisfactory precision in all cases. The products into which the primitive substance is resolved are (by the definition of dissociation) capable of recombining when the conditions that lead to the dissociation are removed. Hence it is by no means easy to determine what proportion of a compound is dissociated under given cir cumstances. When (as in the case of calcium carbonate) the primitive substance and one dis sociation product are solid, and the other dis sociation product is gaseous, the extent of the dissociation may be inferred by computing the mass of the free gas, from observations of its volume, pressure and temperature. When the original substance and the products of dissocia tion are all gaseous, the problem becomes ex ceedingly. difficult, and special means must be devised to fit each special case. In the case of nitrogen tetroxide, N20., which dissociates into the oxide NO., the degree of dissociation may be inferred optically, since N20. is nearly colorless, while NO, has a pronounced brownish-red color. In other cases, the extent of the dissociation may be inferred from observations of the vapor density of the mixture. This method has been
used quite successfully in the case of iodine, the vapor density of this element indicating a molecular formula of I. at temperatures below 700° C. and I at temperatures above 1,500° C.: while at intermediate temperatures the vapor is a mixture of I and I,. The dissociation of acetic acid vapor has been studied by the vapor density method and also by observing the spe cific heat of the mixture at different tempera tures, the specific heat being assumed to be greater (on account of the work done in sep arating the molecules) as the dissociation pro ceeds than it is before the dissociation begins, or after it is complete.
Certain compounds (such as sodium chlo ride) appear to be more or less completely dissociated when they are dissolved in water; but the constituents into which they are re solved possess certain distinctive properties that are not manifested by the resolved parts of sub stances that are dissociated by the action of heat. For this reason it is now usual to dis tinguish this particular phenomenon by a special name, and the dissociation that occurs in such cases is called See ELEcritorrsis;