Liquid line, freezing

The density-temperature diagram for propane is given by Figure 1. The upper, left-hand corner of Figure 1 gives the freezing liquid line. Symbols and units are given in Appendix A, and fixed-point values used for propane are in Table I. 

McCarty, R.D. and Weber, L.A., Thermophysical Properties of Parahydrogen from the Freezing Liquid Line to 5000 R for Pressures to 10,000 psia, Natl. Bur. Stand., Tech. Note 617,1972. 

B—The gas-liquid line always has a positive slope. B negates C. The triple point is below the freezing point. The triple point may be above or below 1 atm. 

The solid/liquid line for an aqueous solution is shown on the phase diagram for water in Fig. 17.12. Since the presence of a solute elevates the boiling point and depresses the freezing point of the solvent, adding a solute has the effect of extending the liquid range. 

If you move to the left (at constant pressure) along the line EF, you will find a temperature at which the liquid freezes. The line AC shows the temperatures and pressures along which solid and liquid are in equilibrium but no vapor is present. If the temperature is decreased further, all of the liquid freezes. Therefore, only solid is present to the left of AC. Water is an unusual substance the solid is less dense than the liquid. If the pressure is increased at point F, at constant temperature, water will melt. The line AC has a slightly negative slope, which is very rare in phase diagrams of other substances. If the pressure on this system is increased and you move up the line AC, you can see that pressure has very little effect on the melting point, so the decrease in temperature is very small. 

Figure 12.8 is the phase diagram for water under moderate pressure. The solid-liquid line leans slightly to the left because < Kcid- The triple point is at 0.01 °C and 611 Pa. The normal freezing point of water is at 0.0002 °C. An increase in pressure decreases the melting... 

There are several additional interesting features of the phase diagram for water. Note that the solid/liquid boundary line has a negative slope. This means that the melting point of water decreases as the external pressure increases. This behavior, which is opposite to that observed for most substances, occurs because the density of ice is less than that of liquid water at the melting point. (The maximum density of water occurs at 4°C.) Thus, when liquid water freezes, its volume increases. Also note that the solid/liquid line has no upper limit. It extends indefinitely, as indicated by the arrow in Figure 16.55. 

The Solid-Liquid Line for Water The phase diagram for water has the same four features but differs from others in one major respect that reveals a key property. Unlike almost any other substance, the solid form is less dense than the liquid that is, water expands upon freezing. Thus, the solid-liquid line has a negative slope (slants to the left with increasing pressure) an increase in pressure converts the solid to the liquid, and the higher the pressure, the lower the temperature at which water freezes (Figure 12.8B). The vertical dashed line at - 1°C crosses the solid-liquid line, which means that ice melts with only an increase in pressure. 

Figure 13.12 Boiling and freezing points of solvent and solution. Phase diagrams of an aqueous solution (dashed lines) and of pure water (solid lines) show that, by lowering the vapor pressure (AP), a solute elevates the boiling point (ATf,) and depresses the freezing point (AT,). (The slope of the solid-liquid line is exaggerated.)...

Why a solution freezes at a lower T. Only solvent vaporizes from solution, so solute molecules are left behind. Similarly, only solvent freezes, again leaving solute molecules behind. The freezing point of a solution is the temperature at which its vapor pressure equals that of the pure solvent, that is, when solid solvent and liquid solution are in equilibrium. The freezing point depression (ATf) occurs because the vapor pressure of the solution is always lower than that of the solvent, so the solution freezes at a lower temperature that is, only at a lower temperature will solvent particles leave and enter the solid at the same rate. In Figure 13.12, the solid-liquid line for the solution is to the left of the pure solvent line at 1 atm and at every other pressure. 

Uquidus curve The freezing point of a molten mixture of substances varies with the composition of the mixture. If the freezing points are plotted as a function of the composition, the line joining the points is called a liquidus curve. Such mixtures usually freeze over a range of temperature. If the temperature at which the last traces of liquid just solidify (assuming that sufficient time has been allowed for equilibrium to be established) are plotted against composition the resulting line is called a solidus curve. 

It is a well-known fact that substances like water and acetic acid can be cooled below the freezing point in this condition they are said to be supercooled (compare supersaturated solution). Such supercooled substances have vapour pressures which change in a normal manner with temperature the vapour pressure curve is represented by the dotted line ML —a continuation of ML. The curve ML lies above the vapour pressure curve of the solid and it is apparent that the vapour pressure of the supersaturated liquid is greater than that of the solid. The supercooled liquid is in a condition of metastabUity. As soon as crystallisation sets in, the temperature rises to the true freezing or melting point. It will be observed that no dotted continuation of the vapour pressure curve of the solid is shown this would mean a suspended transformation in the change from the solid to the liquid state. Such a change has not been observed nor is it theoretically possible. 

The separation of the solid phase does not occur readily with some liquid mixtures and supercooling is observed. Instead of an arrest in the cooling curve at /, the cooling continues along a continuation of c/ and then rises suddenly to meet the line f g which it subsequently follows (Fig. 1,13, 1, iii). The correct freezing point may be obtained by extrapolation of the two parts of the curve (as shown by the dotted line). To avoid supercooling, a few small crystals of the substance which should separate may be added (the process is called seeding ) these act as nuclei for crystallisation. 

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