Phase change freezing

Commercial appHcations of calcium chloride and its hydrates exploit one or more of its properties with regard to aqueous solubiUty, hygroscopic nature, the heat gained or lost when one hydrated phase changes to another, and the depressed freezing point of the eutectic solution at a composition of about 30% by weight calcium chloride. 

We have discovered that a solid can be converted to a liquid by warming it at or above its melting point. Then the solid can be restored merely by recooling. The solid and the liquid are similar in many respects and one is easily obtained from the other. Hence they are called different phases of the same substance. Ice is the solid phase of water and, at room temperature, water is in the liquid phase. The change that occurs when a solid melts or a liquid freezes is called a phase change. 

Let us compare the behavior of these two systems during a phase change. Consider, first, how water acts when it is frozen or vaporized. Pure water freezes at a fixed temperature, 0°C. If we freeze half of a water sample to ice, remove the ice, melt it in another container, and compare the separate samples, we find that the two fractions of the original sample are indistinguishable. 

On the other hand, operations such as distilling or freezing usually tend to separate solutions into the pure substances that were the components of the solution. The nearer alike the components are, the harder it is to separate them from the solution, but even in difficult cases, a variety of methods in succession usually brings about a separation. In nature, solutions are much more common than pure substances, and heterogeneous systems are more common than solutions. When we want pure substances, we often must prepare them from solutions through successive phase changes. 

Changing the pressure will have a similar effect. If we increase p by dp, the solid melts. This process can be reversed at any time by decreasing the pressure by dp. Note that at p = 1 atm (101.325 kPa), only at T = 273.15 K can the phase change be made to occur reversibly because this is the temperature where solid and liquid are in equilibrium at this pressure. If we tried to freeze liquid water aip— atm and a lower temperature such as 263.15 K, the process, once started, would proceed spontaneously and could not be reversed by an infinitesimal change in p or T. 

Solution (a) At 7 — 600 K, liquid Sn freezes at 3 GPa to form solid III. Apparently, no other phase changes occur with increasing pressure, (b) At 550 K, liquid Sn freezes to form solid II at 1.5 GPa, then changes to solid III at 3.5 MPa. (c) At 250 K, solid I converts to solid II at 0.3 GPa, which presumably would convert to solid III at approximately 11 MPa. (The equilibrium line stops at 10 GPa.)... 

A phase change in which the molecules become further separated, such as vaporization, requires energy to break intermolecular attractions and is therefore endothermic. Phase changes that increase molecular contact, such as freezing, are exothermic because energy is given off when attractions form between molecules. 

Phase changes are characteristic of all substances. The normal phases displayed by the halogens appear in Section II-L where we also show that a gas liquefies or a liquid freezes at low enough temperatures. Vapor pressure, which results from molecules escaping from a condensed phase into the gas phase, is one of the liquid properties described in Section II-I. Phase changes depends on temperature, pressure, and the magnitudes of intermolecular forces. 

Phase changes can go in either direction Steam condenses upon cooling, and liquid water freezes at low temperature. Each of these is exothermic because each is the reverse of an endothermic phase change. That is, heat is released as a gas condenses to a liquid and as a liquid freezes to a solid. To make ice cubes, for instance, water... 

Movement across a boundary line corresponds to a phase change. The arrows on the figure show six different phase changes sublimation and its reverse, deposition melting and its reverse, freezing and vaporization and its reverse, condensation. 

In pharmaceutical systems, both heat and mass transfer are involved whenever a phase change occurs. Lyophilization (freeze-drying) depends on the solid-vapor phase transition of water induced by the addition of thermal energy to a frozen sample in a controlled manner. Lyophilization is described in detail in Chapter 16. Similarly, the adsorption of water vapor by pharmaceutical solids liberates the heat of condensation, as discussed in Chapter 17. 

Freeze-drying is a layman s description, and acknowledges that external conditions may alter the conditions of a phase change, i.e. the drying process (removal of water) occurs at a temperature lower than 100°C. 

Figure 5.6 Freeze-drying works by decreasing the pressure, and causing a phase change at higher pressure, the stable form of water is liquid, but the stable form at lower pressures is vapour. Consequently, water (as vapour) leaves a sample when placed in a vacuum or low-pressure chamber we say the sample is freeze-dried ...

Several enthalpies of melting and vaporization are shown in Table 5.1. Notice that the same units (kj/mol) are used for the enthalpies of melting, vaporization, condensation, and freezing. Also notice that energy changes associated with phase changes can vary widely. 

O O When water freezes, the phase change that occurs is exothermic. H20( ) H20(s) ah = -6.02 kj... 

Based on the change in enthalpy, you would expect that water would always freeze. Use the concepts of entropy and free energy to explain why this phase change is favourable only below 0°C. 

Thus far we have restricted our attention to phase changes in which equilibrium is maintained. It also is useful, however, to find procedures for calculating the change in the Gibbs function in transformations that are known to be spontaneous, for example, the freezing of supercooled water at —10°C ... 

The two phases are said to be in equilibrium when the rate at which water molecules entering the solid state is exactly matched by rate entering the liquid state. The temperature at which this occurs is called the melting point, or freezing point, of water. Note that true phase changes are not considered to be chemical reactions as no intramolecular bonds are broken or formed. 

Freezing The phase change during which a liquid is transformed into a solid. 

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