Influence of Pressure on Freezing Points

The experimentally determined activity coefficients, based on vapor pressure, freezing-point and electromotive force measurements, for a number of typical electrolytes of different valence types in aqueous solution at 25 , are represented in Fig. 49, in which the values of log / are plotted against the square-root of the ionic strength in these cases the solutions contained no other electrolyte than the one under consideration. Since the Debye-Htickel constant A for water at 25 is seen from Table XXXV to be 0.509, the limiting slopes of the plots in Fig. 49 should be equal to —0.509 the results to be expected theoretically, calculated in this manner, are shown by the dotted lines. It is evident that the experimental results approach the values required by the Debye-Hiickel limiting law as infinite dilution is attained. The influence of valence on the dependence of the activity coefficient on concentration is evidently in agreement with theoretical expectation. Another verification of the valence factor in the Debye-Hiickel equation will be given later (p. 177). 

References are appended to investigations of the solubility of sodium bromide in organic solvents,2 and to others dealing with such properties of its aqueous solutions as specific heat,3 density,4 refractive index,8 vapour-pressure,6 molecular depression of the freezing-point7 and elevation of the boiling-point,8 electrical constants,9 and the influence of sulphur dioxide on the solubility of the bromide.10... 

Many molecular parameters, such as ionization, molecular and electronic structure, size, and stereochemistry, will influence the basic interaction between a solute and a solvent. The addition of any substance to water results in altered properties for this substance and for water itself. Solutes cause a change in water properties because the hydrate envelopes that are formed around dissolved molecules are more organized and therefore more stable than the flickering clusters of free water. The properties of solutions that depend on solute and its concentration are different from those of pure water. The differences can be seen in such phenomena as the freezing point depression, boiling point elevation, and increased osmotic pressure of solutions. 

What is the direction of the influence of nonideality (for example, positive deviations from Raoult s law) on (a) freezing-point depression, (b) boiling-point elevation, and (c) osmotic pressure compared to the ideal solution case ... 

On this figure two of the phase boundaries have been carefully measured, as the data points show. The two nearly-vertical boundaries at 32°F represent the ordinary freezing curve for water, drawn on the assumption that the small amount of methane that dissolves in water does not influence the shape of this curve much. At high pressures this curve bends to the left (Problem 11.34) but over the range shown it is practically straight and vertical. 

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