242 OUTLINES OF PHYSICAL CHEMISTRY
variable, u, must attain a certain value, which will depend on k and k lf and consequently on the temperature. If, by means of an external pressure, the volume of the carbon dioxide be decreased, the concentration of the gas will be increased, and in order that equilibrium may be re- established a certain quantity of CaO and C0 2 will
In an analogous manner, salts which contain molecules of water of crystallisation exhibit a vapour tension which is solely governed by the temperature. Sodium phosphate (Na 2 HP0 4 .12aq) is an example of this class : at a certain temperature its vapour tension remains constant as long as all the salt has not been transformed into Na 2 HP0 4 .7aq. After this limit has been attained, the vapour tension diminishes, because then it is a different substance which is dissociating.
When, by the dissociation of a solid body, two volatile products are formed, the equation of equilibrium is
and it tells us that the product of the variable concentra- tions (partial pressures) must rise to a certain value, equal
kw to ?r>. an< * dependent on the temperature. This is the
case for ammonium hydrosulphide NH 4 HS, which is
1 Instead of relying on the empirical constant active mass rule, we may assume (as Nemst does in Dammer's « Handbuch,' i. 251) that the so-called non-volatile substances are, in reality, endowed with a certain very small tension of sublimation. In the gaseous system produced by the dissociation of calcium carbonate, the CaCO, and CaO molecules will then be represented by the very small active masses v and ir„ and these will depend only on the temperature (and not at all on the quantities of the residual solid substances).
The concentration of the carbon dioxide will be u = — , and deter-
mined only by the temperature. As -* and ir, are supposed to be very J small, the pressure of the carbon dioxide gas will scarcely be different ] from that of the whole system. The reader will easily find a similar interpretation for some other examples of dissociation referred to in this chapter.
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