The Phase Rule and Its Applications - Alexander Findlay

Temperature.	Pressure of System.
Temperature.	CaCl₂,6H₂O; CaCl₂, 4H₂Oα; vapour.	CaCl₂,6H₂O; CaCl₂, 4H₂Oβ; vapour.
-15°	0.027 cm.	0.022 cm.
0	0.092 ,,	0.076 ,,
+10	0.192 ,,	0.162 ,,
20	0.378 ,,	0.315 ,,
25	0.508 ,,	0.432 ,,
29.2	—	0.567 ,,
29.8	0.680 ,,	—

By reason of the non-recognition of the importance of the solid dissociation product for the definition of the dissociation pressure of a salt hydrate, many of the older determinations lose much of their value.

Suspended Transformation.—Just as in systems of one component we found that a new phase was not necessarily formed when the conditions for its existence were established, so also we find that even when the vapour pressure is lowered below the dissociation pressure of a system, dissociation does not necessarily occur. This is well known in the case of Glauber's salt, first observed by Faraday. Undamaged crystals of Na₂SO₄,10H₂O could be kept unchanged in the open air, although the vapour pressure of the system Na₂SO₄,10H₂O—Na₂SO₄—vapour is greater than the ordinary pressure of aqueous vapour in the air. That is to say, the possibility of the formation of the new phase Na₂SO₄ was given; nevertheless this new phase did not appear, and the system therefore became metastable, or unstable with respect to the anhydrous salt. When, however, a trace of the new phase—the anhydrous salt—was brought in contact with the hydrate, transformation occurred; the hydrate effloresced.

The possibility of suspended transformation or the non-formation of the new phases must also be granted in the case where the vapour pressure is raised above that corresponding to the system hydrate—anhydrous salt (or lower hydrate)—vapour; in this case the formation of the higher hydrate becomes a possibility, but not a certainty. Although there is no example of this known in the case of hydrated salts, the suspension of the transformation has been observed in the case of the compounds of ammonia with the metal chlorides (p. [82]). Horstmann,[^[158]] for example, found that the pressure of ammonia in contact with 2AgCl,3NH₃ could be raised to a value higher than the dissociation pressure of AgCl,3NH₃ without this compound being formed. We see, therefore, that even when the existence of the higher compound in contact with the lower became possible, the higher compound was not immediately formed.

Range of Existence of Hydrates.—In Fig. 19 the vapour

pressure curves of the different hydrates of copper sulphate are represented as maintaining their relative positions throughout the whole range of temperatures. But this is not necessarily the case. It is possible that at some temperature the vapour pressure curve of a lower hydrate may cut that of a higher hydrate. At temperatures above the point of intersection, the lower hydrate would have a higher vapour pressure than the higher hydrate, and would therefore be metastable with respect to the latter. The range of stable existence of the lower hydrate would therefore end at the point of intersection. This appears to be the case with the two hydrates of sodium sulphate, to which reference will be made later.[^[159]]

Constancy of Vapour Pressure and the Formation of Compounds.—We have seen in the case of the salt hydrates that the continued addition of the vapour phase to the system caused an increase in the pressure until at a definite value of the pressure a hydrate is formed; the pressure then becomes constant, and remains so, until one of the solid phases has disappeared. Conversely, on withdrawing the vapour phase, the pressure remained constant so long as any of the dissociating compound was present, independently of the degree of the decomposition (p. [86]). This behaviour, now, has been employed for the purpose of determining whether or not definite chemical compounds are formed. Should compounds be formed between the vapour phase and the solid, then, on continued addition or withdrawal of the vapour phase, it will be found that the vapour pressure remains constant for a certain time, and will then suddenly assume a new value, at which it will again remain constant. By this method, Ramsay[^[160]] found that no definite hydrates were formed in the case of ferric and aluminium oxides, but that two are formed in the case of lead oxide, viz. 2PbO,H₂O and 3PbO,H₂O.

The method has also been applied to the investigation of the so-called palladium hydride,[^[161]] and the results obtained appear to show that no compound is formed. Reference will, however, be made to this case later (Chap. X.).