Lakhasly

4.6.3 REDUCING THE SOLUTION FREEZING TEMPERATURE The phase diagram of pure water was previously studied.The quantitative effect of concentration on lowering the freezing temperature of aqueous solutions is described by the following expression: AT = Kk(H20) o Cm where Kk(H20) - is the cryoscopic constant of the solvent.In accordance with Raoul's law, the saturated vapor pressure above the solution decreases; in the phase diagram, this leads to the fact that the triple point moves not only down (to the region of lower pressures), but also to the left.In the pressure-temperature coordinates, the diagram shows the state of aggregation of water and the conditions under which phase transitions occur - melting and boiling of a liquid.The freezing (curing) of the liquid solution occurs at the triple point (indicated by the letter O in Fig.4.20.4.20.

4.6.3 REDUCING THE SOLUTION FREEZING TEMPERATURE
The phase diagram of pure water was previously studied. In the pressure-temperature coordinates, the diagram shows the state of aggregation of water and the conditions under which phase transitions occur - melting and boiling of a liquid.
Now we consider the changes in the phase diagram that occur during the transition from pure water to an aqueous solution, Fig. 4.20.
solution boiling temperature
T.R
solution freezing temperature
Fig. 4.20. Comparison of phase diagrams of pure water (solid lines) and aqueous solution (dashed lines).
The freezing (curing) of the liquid solution occurs at the triple point (indicated by the letter O in Fig. 20), in which the liquid, vapor and the solid phase are in equilibrium. In accordance with Raoul's law, the saturated vapor pressure above the solution decreases; in the phase diagram, this leads to the fact that the triple point moves not only down (to the region of lower pressures), but also to the left. This circumstance means that the freezing temperature of the solution should be less than the freezing temperature of a pure solvent.
In the case of water, the freezing temperature of aqueous solutions becomes lower than 0°C. The quantitative effect of concentration on lowering the freezing temperature of aqueous solutions is described by the following expression:
AT = Kk(H20) • Cm
where Kk(H20) - is the cryoscopic constant of the solvent. Kkr is different for different liquids and does not depend on the nature of the solute; for water, Kk(H20) is 1,86 K•kg/mol. Cm- - molar concentration of the solution (mol of sol. substance / kg of solvent)