Part 10 (1/2)
As the temperature of the saturated solution falls, silver nitrate is deposited, and on lowering the temperature sufficiently a point will at last be reached at which ice also begins to separate out. Since there are now four phases co-existing, viz. silver nitrate, ice, solution, vapour, the system is invariant, and the point is a _quadruple point_. This quadruple point, therefore, forms the lower limit of the solubility curve of silver nitrate. Below this point the solution becomes metastable.
Ice as Solid Phase.--Ice melts or is in equilibrium with water at a temperature of 0. The melting point, will, however, be lowered by the solution of silver nitrate in the water; and the greater the concentration of the salt in the solution the greater will be the depression of the temperature of equilibrium. On continuing the addition of silver nitrate, a point will at length be reached at which the salt is no longer dissolved, but remains in the solid form along with the ice. We again obtain, therefore, the invariant system ice--salt--solution--vapour. The temperature at which this invariant system can exist has been found by Middelberg[193] to be -7.3, the solution at this point containing 47.1 per cent. of silver nitrate.
The same general behaviour will be found in the case of all other systems of two components belonging to this cla.s.s; that is, in the case of systems from which the components crystallise out in the pure state, and in which the fused components are miscible in all proportions. In all such cases, therefore, the solubility curves (curves of equilibrium) can be represented diagrammatically as in Fig. 29. In this figure OA represents the solubility curve of the salt, and OB the freezing {117} point curve of ice. O is the quadruple point at which the invariant system exists, and may be regarded as the point of intersection of the solubility curve with the freezing-point curve. Since this point is fixed, the condition of the system as regards temperature, vapour pressure, and concentration of the components (or composition of the solution), is perfectly definite. From the way, also, in which the condition is attained, it is evident that the quadruple point is the lowest temperature that can be obtained with mixtures of the two components in presence of vapour. It is known as the _cryohydric point_, or, generally, the _eutectic point_.[194]
[Ill.u.s.tration: FIG. 29.]
Cryohydrates.[195]--On cooling a solution of common salt in water to a temperature of -3, Guthrie observed that the hydrate NaCl,2H_{2}O separated out. This salt continued to be deposited until at a temperature of -22 opaque crystals made their appearance, and the liquid pa.s.sed into the solid state without change of temperature. A similar behaviour was found by Guthrie in the case of a large number of other salts, a temperature below that of the melting point of ice being reached at which on continued withdrawal of heat, the solution solidified at a constant temperature. When the system had attained this minimum temperature, it was found that the composition of the solid and the liquid phases was the same, and remained unchanged throughout the period of solidification. This is shown by the following figures, which give the composition of different samples of the solid phase deposited from the solution at constant temperature.[196]
{118}
--------------------------------- No.
Temperature of
NaCl.
solidification.
Per cent.
----
-----------------
---------- 1
-21 to -22
23.72 2
-22
23.66 3
-22
23.73 4
-23
23.82 5
-23
23.34 6
-23
23.35 --------------------------------- Mean 23.6 ---------------------------------
Conversely, a mixture of ice and salt containing 23.6 per cent. of sodium chloride will melt at a definite and constant temperature, and exhibit, therefore, a behaviour supposed to be characteristic of a pure chemical compound. This, then, combined with the fact that the solid which was deposited was crystalline, and that the same constant temperature was attained, no matter with what proportions of water and salt one started, led Guthrie to the belief that the solids which thus separated at constant temperature were definite chemical compounds, to which he gave the general name _cryohydrate_. A large number of such cryohydrates were prepared and a.n.a.lysed by Guthrie, and a few of these are given in the following table, together with the temperature of the cryohydric point:[197]--
CRYOHYDRATES.
------------------------------------------------------------------ Salt.
Cryohydric point.
Percentage of anhydrous
salt in the cryohydrate.
------------------------------------------------------------------ Sodium bromide
-24
41.33 Sodium chloride
-22
23.60 Pota.s.sium iodide
-22
52.07 Sodium nitrate
-17.5
40.80 Ammonium sulphate
-17
41.70 Ammonium chloride
-15
19.27 Sodium iodide
-15
59.45 Pota.s.sium bromide
-13
32.15 Pota.s.sium chloride
-11.4
20.03 Magnesium sulphate