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Binary Phase diagram: The Calcium Chloride - water system
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 a eutectic point, also called a cryohydric or cryohydratic point.
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| Phase diagram for the binary CaCl2 - H2O system at temperatures below 100°C |
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 will be the hexahydrate, CaCl2·6H2O.
Between B and C the solid phase will be the tetrahydrate, CaCl2·4H2O. At higher concentrations the dihydrate, CaCl2·2H2O and later the anhydrate CaCl2 will 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 above. 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.