Equation 47 suggests that freezing-point depression is a solvent property. This observa-
tion has been formalized in the definition of a cryoscopic constant, Kf, for a given solvent:
∆T = Kf mB
(48)
which for water is 1.86 K kg mol1.
The freezing point is actually a phase-equilibrium line along which, as the temperature of
a system decreases, a largely pure ice phase is in equilibrium with increasingly concen-
trated aqueous solutions. Examples of this behavior can be seen in Figures 11 and 12, which
0
NaCl Hydrolite Limb
5
NaCl Ice Limb
10
15
20
25
0.0
1.0
2.0
3.0
4.0
5.0
6.0
(moles kg1)
Figure 11. Phase diagram for NaCl-H2O system. (Courtesy of Dr. G.M. Marion, USACRREL.)
900
801C
800
700
Liquid
600
500
NaCl + Liquid
400
300
200
CaCl2 6H2O + Liquid
Ice + Liquid
100
NaCl 2H2O + NaCl
Figure 12. Phase diagram for CaCl2-
0
H2O system. (Courtesy of Dr. G.M.
Ice + CaCl2 6H2O
Marion, USACRREL.)
100
H2O
20
40
60
80
NaCl
Mass (%)
17
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