Aqueous Salt Solutions


Binary Phase diagram: The Calcium Chloride - water system Calcium chloride phase diagram

Pure water freezes to ice at 0°C. If CaCl2 or another solute is added to water, the freezing point of the solution will be lower than 0°C. As the graph below shows, ice will form at -20°C in a 20 mass % solution of CaCl2. This phenomenon is called a freezing point depression. It can be explained from changes in chemical potentials. A solution containing 31 mass % CaCl2 has the lowest freezing point of any CaCl2 solution (about -50°C) . This solution is called a eutectic solution. The point (A) in the diagram marking the freezing point of this solution is an eutectic point, also called a cryohydric or cryohydratic point.

At the freezing point of an eutectic CaCl2 solution, the solution is in equilibrium with two solid phases: ice and CaCl2·6H2O. Counting also the vapor phase, a total of four phases are in equilibrium in this binary system. The eutectic point therefore constitutes an invariant point. On the curve between the points A and B the solid phase precipitating from saturated solutions is the hexahydrate, CaCl2·6H2O. Between B and C the stable solid phase is the tetrahydrate, CaCl2·4H2O. At higher concentrations the dihydrate, CaCl2·2H2O and later the anhydrate CaCl2 precipitate (not shown in the diagram).

The points B and C are peritectic points. In a peritectic point, a solid phase changes upon heating into a liquid in equilibrium with another solid phase. CaCl2·4H2O consists of 60.6 mass percent CaCl2. This composition is marked with a vertical green line in the figure. If CaCl2·4H2O is heated to 45.3°C it will decompose into a liquid in equilibrium with CaCl2·2H2O. The temperature at which this happens is marked with a horizontal green line. The two points B and C represent solutions in equilibrium with two solid phases and a gas phase and therefore constitute invariant points.

A 45 wt% CaCl2 solution at 20°C consists of saturated aqueous solution and solid CaCl2·6H2O. If the solution contains 55% CaCl2 the equilibrium mixture no longer contains aqueous solution but only the two solids, CaCl2·6H2O and CaCl2·4H2O. If this mixture is heated slightly above the peritectic temperature B, the CaCl2·6H2O melts and the remaining mixture is CaCl2·4H2O in equilibrium with aqueous solution.