The Process of Crystallization
This section will briefly review the physico-chemical processes that occur during a freezing process. The figure below shows the time-temperature relationship for freezing of pure water (ABCDE) and aqueous solutions (AB'C'D'). The first thermal event that can be seen from such a diagram is undercooling below the freezing point before the induction of crystallization, from A to B or B' . This is a non-equilibrium, metastable state which is analogous to an activation energy necessary for the nucleation process. Pure water can be undercooled by several degrees before the nucleation phenomenon begins.
Once the critical mass of nuclei is reached, the system nucleates at point B or B' in the figure and releases its latent heat faster than heat is being removed from the system. In aqueous solutions, however, B' is not as low as B, since the added solute will promote heterogeneous nucleation, thereby accelerating the nucleation process. The temperature increases instantly to the initial freezing temperatureof the solution at Point C (0oC) or C' (Tf). The presence of solutes results in depression of the freezing point based on Raoult's Law, which relates vapor pressure of the solution to that of pure solvent based on solute concentration. Note that C' is not as high as C, because the initial freezing point is depressed as a result of the solute. Hence, the solute has greatly decreased the amount of undercooling for two reasons: faster nucleation and lowered freezing point. In very concentrated solutions, it is sometimes even difficult to induce undercooling.
In pure water, the time line from C to D in the figure reflects the time during which crystal growth is occurring at 0oC. Fast freezing rates promote the formation of many small ice crystals during this period. The partially frozen mixture will not cool until all of the "freezable" water has crystallized; hence, the line CD for pure water occurs at constant temperature. The freezing time is usually defined as the time from the onset of nucleation to the end of the crystal growth phase. After crystallization is completed, the temperature drops from D to E as sensible heat of ice is removed.
During the freezing of the aqueous solution, a freeze-concentration process occurs as water freezes out of solution in the form of pure ice crystals (C'D'), effectively removing solvent from the solute. Hence the freezing temperature of the remaining solution continues to drop. At temperatures well below the initial freezing point, some liquid water remains. Also, a large increase in the viscosity of the unfrozen phase occurs, thus decreasing the diffusion properties of the system and hindering crystallization. It is more difficult to assign a freezing time to this process, but it is usually taken as the time to reach some predetermined temperature below the initial freezing point. This freeze-concentration process establishes the freezing curve.