Liquid solutions sublimation pressure curve

An increase in the boiling point or a decrease in the freezing point of a solution containing a nonvolatile component is compared to pure solvent caused by a reduction in the vapor pressure. The reduction of the freezing point AT of a solution is T — T, as shown in Fig. 1-42 where T is the freezing point (or melting point) of the pure solvent. At Tg the vapor pressure is the same for the liquid and solid phases of the solvent. Tg is defined by the intersection A of the vapor pressure curve VC and the sublimation pressure curve SC of the solvent. If only pure solvent freezes, the freezing point T of the solution occurs at the intersection B of the vapor pressure curve of the solution VCS and the sublimation pressure curve of the solvent SC. 

Another consequence of lowering of vapour pressure is that the freezing point of the solution is lower than that of the pure solvent. The freezing point of a solution is the temperature at which the solution exists in equilibrium with solid solvent. In such an equilibrium, the solvent must have the same vapour pressure in both solid and liquid states. Consequently, the freezing point is the temperature at which the vapour pressure curves of the solvent and solution intersect the sublimation curve of the solid solvent that is, points C and D, respectively, in Fig. 2.8. The freezing point depression is T-To = ATf. An expression for freezing... 

The relations which are found here will be best understood with the help of Fig. 72 In this figure, OB represents the sublimation curve of ice, and BC the vaporisation curve of water the curve for the solution must lie below this, and must cut the sublimation curve of ice at some temperature below the melting-point. The point of intersection A is the cryohydric point. If the solubility increases with rise of temperature, the increase of the vapour pressure due to the latter will be partially annulled. Since at first the effect of increase of temperature more than counteracts the depressing action of increase of concentration, the vapour pressure will increase on raising the temperature above the cryohydric point. If the elevation of temperature is continued, however, to the melting-point of the salt, the effect of increasing concentration makes itself more and more felt, so that the vapour-pressure curve of the solution falls more and more below that of the pure liquid, and the pressure will ultimately become equal to that of the pure salt that is to say, practically equal to zero. The curve will therefore be of the general form AMF shown in Fig. 72. If the solubility should diminish with rise of temperature, the two factors, temperature and concentration, will act in the same direction, and the vapour-pressure curve will rise relatively more rapidly than that of the pure liquid since, however, the pure salt is ultimately obtained, the vapour-pressure curve must in this case also finally approach the value zero.2... 

Corresponding to the point Q/the melting-point of pure iodine, there is the point C, which represents the vapour pressure of iodine at its melting-point. At this point three curves cut i, the sublimation curve of iodine 2, the vaporisation curve of fused iodine 3, CiB, the vapour-pressure curve of the saturated solutions in equilibrium with solid iodine. Starting, therefore, with the system solid iodine— liquid iodine, addition of chlorine will cause the temperature of equilibrium to fall continuously, while the vapour pressure will first increase, pass through a maximum and then fall continuously until the eutectic point, B (Bjl), is reached. At this point the system is invariant, and the pressure will therefore remain constant until all the iodine has disappeared. As the concentration of the chlorine increases in the manner represented by the curve B/H, the pressure of the vapour also increases as represented by the curve Bj/iHi. At the eutectic point for iodine monochloride and iodine trichloride, the pressure again remains constant until all the monochloridc has disappeared. As the concentration of the solution passes along the curve HF, the pressure... 

When a solution freezes, the solid is usually pure solvent. Thus the solid-vapor equilibrium (sublimation) P-T curve is unaffected by the presence of solute. The intersection of this curve and the liquid-vapor curve is the triple point (nearly the same temperature as the freezing point, which is measured at atmospheric pressure). Since a solute lowers the solvent vapor pressure, the triple point is shifted to lower temperature, as shown in Figure 11-2. Detailed calculations show that the decrease in freezing point for a dilute solution is proportional to the total molal concentration of solutes... 

As described in Figure 4b the phase behavior of a type II binary system is depicted by the vapor pressure (L-V boundary) curves for the pure components, sublimation (S-V boundary) and melting (S-L boundary) curves for the solid component, and especially the S-L-V line on the P-T space. For an organic solid drug solute, the triple-point temperature is sufficiently higher than the critical temperature of the SCF solvent. The (L = V) critical locus has two branches and is intersected by two S-L-V lines at LfCEP and LCEP, respectively, in the presence of the solid phase. The S-L-V line indicates that the melting of the solid is lowered in the presence of the SCF solvent component as it is dissolved in the molten (liquid) phase. The S-L-V line... 

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