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. 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 an 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. An invariant point has no degrees of freedom - one specific temperature, pressure, and concentration is required.

The curve between the points A and B marks compositions of solutions that are saturated with the hexahydrate, CaCl2·6H2O. 

The curve between B and C marks compositions of solutions that are saturated with the tetrahydrate, CaCl2·4H2O. The dihydrate CaCl2·2H2O is in equilibrium with saturated solutions at temperatures higher than that marked by point C. At even higher temperatures, saturated solutions are in equilibrium with the anhydrate, CaCl2,  (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.

The 25°C isotherm is marked with a dashed, black line in the diagram. Between h and i the mixture is liquid. No solid precipitates. Point i marks the interception between the dashed line and the solubility curve. In this point, the solution is saturated with CaCl2·6H2O, but no precipitate has formed. Between i and j, increasing amounts of hexahydrate, CaCl2·6H2O is precipitating. Some liquid with a composition at i remains in equilibrium with this solid. In point j, the composition of the solution corresponds to the composition of CaCl2·6H2O. At this point, there is no liquid but only solid CaCl2·6H2O. Between j and k, mixtures consist of CaCl2·6H2O (s) and CaCl2·4H2O (s), no liquid is present. Between k and l, mixtures consist of CaCl2·4H2O (s) and CaCl2·2H2O (s), no liquid is present. Mixtures with CaCl2 concentrations higher than that marked by l consist of CaCl2·2H2O (s) and CaCl2(s).

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