Chapter V. the behaviour of such hydrates was more fully studied in the
light of the Phase Rule. Glauber's salt, or sodium sulphate decahydrate, for example, on being heated to a temperature of about 32.5deg, partially liquefies, owing to the fact that the water of crystallization is split off and anhydrous sodium sulphate formed, as shown by the equation--
Na_{2}SO_{4},10H_{2}O = Na_{2}SO_{4} + 10H_{2}O
The temperature of 32.5deg, it was learned, constituted a _transition point_ for the decahydrate and anhydrous salt plus water; decomposition of the hydrated salt occurring above this temperature, combination of the anhydrous salt and water below it.
Analogous phenomena are met with in systems constituted of two salts and water in which the formation of double salts can take place. Thus, for example, if _d_-sodium potassium {259} tartrate is heated to above 55deg, apparent partial fusion occurs, and the two single salts, _d_-sodium tartrate and _d_-potassium tartrate, are deposited, the change which occurs being represented by the equation--
4NaKC_{4}O_{6}H_{4},4H_{2}O = 2Na_{2}C_{4}O_{6}H_{4},2H_{2}O + 2K_{2}C_{4}O_{6}H_{4},1/2H_{2}O + 11H_{2}O
On the other hand, if sodium and potassium tartrates are mixed with water in the proportions shown on the right side of the equation, the system will remain partially liquid so long as the temperature is maintained above 55deg (in a closed vessel to prevent loss of water), but on allowing the temperature to fall below this point, complete solidification will ensue, owing to the formation of the hydrated double salt. Below 55deg, therefore, the hydrated double salt is the stable system, while above this temperature the two single salts plus saturated solution are stable.[337]
A similar behaviour is found in the case of the double salt copper dipotassium chloride (CuCl_{2},2KCl,2H_{2}O or CuK_{2}Cl_{4},2H_{2}O).[338] When this salt is heated to 92deg, partial liquefaction occurs, and the original blue plate-shaped crystals give place to brown crystalline needles and white cubes; while on allowing the temperature to fall, re-formation of the blue double salt ensues. The temperature 92deg is, therefore, a transition point at which the reversible reaction--
CuK_{2}Cl_{4},2H_{2}O <--> CuKCl_{3} + KCl + 2H_{2}O
takes place.
The decomposition of sodium potassium tartrate, or of copper dipotassium chloride, differs in so far from that of Glauber's salt that _two_ new solid phases are formed; and in the case of copper dipotassium chloride, one of the decomposition products is itself a double salt.
In the two examples of double salt decomposition which have just been mentioned, sufficient water was yielded to cause a partial liquefaction; but other cases are known where this is not so. Thus, when copper calcium acetate is heated to a {260} temperature of 75deg, although decomposition of the double salt into the two single salts occurs as represented by the equation[339]--
CuCa(C_{2}H_{3}O_{2})_{4},8H_{2}O = Cu(C_{2}H_{3}O_{2})_{2},H_{2}O + Ca(C_{2}H_{3}O_{2})_{2},H_{2}O + 6H_{2}O
the amount of water split off is insufficient to give the appearance of partial fusion, and, therefore, only a change in the crystals is observed.
The preceding examples, in which decomposition of the double salt was effected by a rise of temperature, were chosen for first consideration as being more analogous to the case of Glauber's salt; but not a few examples are known where the reverse change takes place, formation of the double salt occurring _above_ the transition point, and decomposition into the constituent salts below it. Instances of this behaviour are found in the case of the formation of astracanite from sodium and magnesium sulphates, and of sodium ammonium racemate from the two sodium ammonium tartrates, to which reference will be made later. Between these various systems, however, there is no essential difference; and whether decomposition or formation of the double salt occurs at temperatures above the transition point, will of course depend on the heat of change at that point. For, in accordance with van't Hoff's law of movable equilibrium (p. 58), that change will take place at the higher temperature which is accompanied by an absorption of heat. If, therefore, the formation of the double salt from the single salts is accompanied by an absorption of heat, the double salt will be formed from the single salts on raising the temperature; but if the reverse is the case, then the double salt on being heated will decompose into the constituent salts.[340]
In those cases, now, which have so far been studied, the change at the transition point is accompanied by a taking up or a splitting off of water; and _in such cases the general rule can be given, that if the water of crystallization of the two constituent {261} salts together is greater than that of the double salt, the latter will be produced from the former on raising the temperature_ (_e.g._ astracanite from sodium and magnesium sulphates); _but if the double salt contains more water of crystallization than the two single s