![]() After crystallization is completed, the temperature drops from D to E as sensible heat of ice is removed.ĭuring 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. The freezing time is usually defined as the time from the onset of nucleation to the end of the crystal growth phase. 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. ![]() Fast freezing rates promote the formation of many small ice crystals during this period. In pure water, the time line from C to D in the figure reflects the time during which crystal growth is occurring at 0 oC. In very concentrated solutions, it is sometimes even difficult to induce undercooling. Hence, the solute has greatly decreased the amount of undercooling for two reasons: faster nucleation and lowered freezing point. Note that C’ is not as high as C, because the initial freezing point is depressed as a result of the solute. 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. ![]() The temperature increases instantly to the initial freezing temperature of the solution at Point C (0 oC) or C’ (T f). In aqueous solutions, however, B’ is not as low as B, since the added solute will promote heterogeneous nucleation, thereby accelerating the nucleation process. 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. The figure shows the time-temperature relationship for freezing of pure water (ABCDE) and aqueous solutions (AB’C’D’). Pure water can be undercooled by several degrees before the nucleation phenomenon begins. This is a non-equilibrium, metastable state which is analogous to an activation energy necessary for the nucleation process. 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’. The figure below shows the time-temperature relationship for freezing of pure water (ABCDE) and aqueous solutions (AB’C’D’). This section will briefly review the physico-chemical processes that occur during a freezing process.
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