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The definite vapour pressure over a hydrated salt, as compared with the variable pressure over a solution, enables us to distinguish between the two cases. A mechanical mixture of liquid water with a solid has a vapour pressure equal to that of pure water, provided a solution is not formed. A hydrate containing hygroscopic moisture, in excess of its combined amount, will show a vapour pressure equal to that of its saturated solution, until all the excess of moisture has been lost; the pressure will then drop abruptly to that of the definite solid hydrate, say CuSO4,5H2O, and the pressure falls to A.
 | Fig: Hydrates vapour pressure Vapour pressure curves for dissociation of a series of hydrates of copper sulphate at 50°. |
Dissociation of this hydrate then begins: CuSO4,5H2O <=> CuSO4,3H2O + 2H2O, and the system composed of the two solid hydrates, CuSO4,5H2O and CuSO4,3H2O, has, in accordance with the Phase Rule, a definite pressure. With continued abstraction of water, all the CuSO4,5H2O, is converted into CuSO4,3H2O, and the pressure again falls abruptly to a lower value, C.
Dissociation into CuSO4,H2O now begins: CuSO4,3H2O <=> CuSO4,H2O + 2H2O. This hydrate has a very small vapour pressure, but gives off water in a desiccator over phosphorus pentoxide, forming the anhydrous salt. When all the trihydrate is converted into the monohydrate, the pressure falls sharply to a low value £, and remains S this pressure until all the water is removed; it then falls to zero over the anhydrous salt: CuSO4,H2O <=> CuSO4 + H2O. By analysing the solid when the sudden drops of pressure occur, say at C, the composition of the hydrates may be found.
The dotted curve AO represents the vapour pressure of a solid solution (e.g., jelly), which loses water continuously.
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