Let us now follow the changes which take place on
increasing the pressure of the aqueous vapour in contact with anhydrous copper sulphate, the temperature being meanwhile maintained constant. If, starting from the point D, we slowly add water vapour to the system, the pressure will gradually rise, without formation of hydrate taking place; for at pressures below the curve OC only the anhydrous salt can exist. At E, however, the hydrate CuSO4,H2O will be formed, and as there are now three phases present, viz. CuSO4, CuSO4,H2O, and vapour, the system becomes univariant; and since the temperature is constant, the pressure must also be constant. Continued addition of vapour will result merely in an increase in the amount of the hydrate, and a decrease in the amount of the anhydrous salt. When the latter has entirely disappeared, i.e. has passed into hydrated salt, the system again becomes bivariant, and passes along the line EF; the pressure gradually increases, therefore, until at F the hydrate 3H2O is formed, and the system again becomes univariant; the three phases present are CuSO4,H2O, CuSO4,3H2O, vapour. The pressure will remain constant, therefore, until the hydrate 1H2O has disappeared, when it will again increase till G is reached; here the hydrate 5H2O is formed, and the pressure once more remains constant until the complete disappearance of the hydrate 3H2O has taken place.
Conversely, on dehydrating CuSO4,5H2O at constant temperature, we should find that the pressure would maintain the value corresponding to the dissociation pressure of the system CuSO4,5H2O—CuSO4,3H2O—vapour, until all the hydrate 5H2O had disappeared; further removal of water would then cause the pressure to fall abruptly to the pressure of the system CuSO4,3H2O—CuSO4,H2O—vapour, at which value it would again remain constant until the tri-hydrate had passed into the monohydrate, when a further sudden diminution of the pressure would occur. This behaviour is represented diagrammatically in Fig. 20, the values of the pressure being those at 50°.
Efflorescence.—From Fig. 19 we are enabled to predict the conditions under which a given hydrated salt will effloresce when exposed to the air. We have just learned that copper
sulphate pentahydrate, for example, will not be formed unless the pressure of the aqueous vapour reaches a certain value; and that conversely, if the vapour pressure falls below the dissociation pressure of the pentahydrate, this salt will undergo dehydration. From this, then, it is evident that a crystalline salt hydrate will effloresce when exposed to the air, if the partial pressure of the water vapour in the air is lower than the dissociation pressure of the hydrate. At the ordinary temperature the dissociation pressure of copper sulphate is less than the pressure of water vapour in the air, and therefore copper sulphate does not effloresce. In the case of sodium sulphate decahydrate, however, the dissociation pressure is greater than the normal vapour pressure in a room, and this salt therefore effloresces.
Indefiniteness of the Vapour Pressure of a Hydrate.—Reference has already been made (p. [84]), in the case of the ammonia compounds of the metal chlorides, to the importance of the solid product of dissociation for the definition of the dissociation pressure. Similarly also in the case of a hydrated salt. A salt hydrate in contact with vapour constitutes only a bivariant system, and can exist therefore at different values of temperature and pressure of vapour, as is seen from the diagram, Fig. 19. Anhydrous copper sulphate can exist in contact with water vapour at all values of temperature and pressure lying in the field below the curve OC; and the hydrate CuSO4,H2O can exist in contact with vapour at all values of temperature and pressure in the field BOC. Similarly, each of the other hydrates can exist in contact with vapour at different values of temperature and pressure.
From the Phase Rule, however, we learn that, in order that at a given temperature the pressure of a two-component system
may be constant, there must be three phases present. Strictly, therefore, we can speak only of the vapour pressure of a system; and since, in the cases under discussion, the hydrates dissociate into a solid and a vapour, any statement as to the vapour pressure of a hydrate has a definite meaning only when the second solid phase produced by the dissociation is given. The everyday custom of speaking of the vapour pressure of a hydrated salt acquires a meaning only through the assumption, tacitly made, that the second solid phase, or the solid produced by the dehydration of the hydrate, is the next lower hydrate, where more hydrates than one exist. That a hydrate always dissociates in such a way that the next lower hydrate is formed is, however, by no means certain; indeed, cases have been met with where apparently the anhydrous salt, and not the lower hydrate (the existence of which was possible), was produced by the dissociation of the higher hydrate.[[156]]
That a salt hydrate can exhibit different vapour pressures according to the solid product of dissociation, can not only be proved theoretically, but it has also been shown experimentally to be a fact. Thus CaCl2,6H2O can dissociate into water vapour and either of two lower hydrates, each containing four molecules of water of crystallization, and designated respectively as CaCl2,4H2Oα, and CaCl2,4H2Oβ. Roozeboom[[157]] has shown that the vapour pressure which is obtained differs according to which of these two hydrates is formed, as can be seen from the following figures:—