HYDRATES. 189
- THERMIC ANALYSIS. If the freezing-point of an aqueous solution of a
salt is determined at various concentrations and the results are plotted with
the temperatures as ordinates and with the concentrations in per cent as
abscissas, a curve AC is obtained which starts at the freezing-point of pure
water and descends in accordance with the law of Raoult and van't Hoff
(Fig. 23). The highly concentrated solutions, on the other hand, are to be
regarded as solutions of water in the salt; the curve CB shows the region in
which the freezing-point rises as the amount of water diminishes until finally
the freezing-point (or melting-point) of the pure salt is reached. In the
region of the first curve, that part of the mixture which acts as solvent
crystallizes out on freezing, — in this case pure ice; in the region of the sec-
ond curve, the solid salt is deposited as the solution cools. The point where
Oonc.
FIG. 23.Compouijd Cone.
FIG. 24.these two curves intersect is called the eutectic point, and is characterized by
the fact that salt and ice crystallize simultaneously in an intimate, eutectic
mixture. Such mixtures, which are also known as cryohydrates, possess
freezing (or melting) points which are constant, and are lower than the freez-
ing-point of pure water; they are on this account used for maintaining uniform
temperatures of below zero centigrade.
If the salt employed forms a chemical compound with water, then the two
systems water/hydrate on the one hand and hydrate/anhydrous salt on the
other hand are each to be considered independently, according to the principle
just outlined. If, as before, the concentrations are plotted as abscissas and
the temperatures as ordinates, a double pair of curves is obtained with two
eutectic points. The two inside curves unite in a common maximum which
is the freezing-point of the pure hydrate (Fig. 24). Conversely it is clear
that by determining the freezing-points of a complete series of mixtures of
the salt and water, the presence of a hydrate can be detected by the appear-
ance of a maximum on the curve, and its composition can be read by dropping
a perpendicular from the maximum point to the horizontal axis. Several
maximum points indicate the presence of several hydrates. In this way
Roozeboom has, for example, proved in the case of ferric chloride the existence
of the hydrates 2FeCl 3 -4H 2 O. 2FeCl 3 -5H 2 O, 2FeCl 3 -7H 2 O, 2FeCl 3 -12H 2 O.