FREEZING POINT OF A DILUTE SOLUTION 197 



In a mixture of ice and water at 0° C, the less dense ice (0.91674 

 gram/cc) floats on the water (0.99987 gram/cc). Since water expands 

 on solidification, increasing the pressure will lower the freezing point. 

 If the material contracts on freezing, like paraffin, then an increase in 

 pressure will raise the freezing point. For water, an increase in pressure 

 of 1 atmosphere will lower the freezing point 0.0075° C. Therefore, in 

 making freezing-point measurements which involve water as a solvent, 

 the very minute correction due to changes in atmospheric pressure can 

 be disregarded. 



The freezing point of any solution is invariably lower than that of the 

 pure solvent. The temperature at which a solution freezes is lower the 

 greater the amount of substance dissolved. When such a solution begins 

 to freeze, it is only the solvent which freezes out at first. As a result, 

 the remainder of the solute is more concentrated. The freezing point of 

 this concentrate is in turn still lower. The process continues until the 

 solution becomes saturated. At this point the dissolved substance and 

 the solvent freeze out so that the concentration of both remains un- 

 changed during solidification. 



Freezing Point of a Dilute Solution of a Non-Electrolyte 



When a solid like sugar is dissolved in water to form a dilute solution 

 the freezing point of the solution is below the freezing point of pure 

 water. We refer to this lowered freezing temperature as the depressed 

 freezing point. It is designated by A and is expressed in terms of the 

 number of Centigrade degrees below the freezing point of distilled water 

 taken as zero. Experimentally it is found that a solution containing 

 3.42 grams of sucrose dissolved in 1000 grams of water, i.e., a concen- 

 tration of 0.01 M, freezes at —0.0186° C. The depression of the 

 freezing point A = 0.0186. As the concentration is decreased, the 

 freezing-point depression is proportionally decreased. At concentra- 

 tion 0.001 (Table V-7), A = 0.00186, and proportionally lower values 

 are obtained as the concentration decreases until at infinite dilution 

 (zero concentration) the freezing-point depression is zero. 



If other non-electrolytes are dissolved in water and examined in a 

 similar manner, it is found that all those solutions containing 0.01 

 gram-molecular weight per 1000 grams of water have a common freezing 

 point, namely, —0.0186° C. They all possess A values which are smaller 

 for proportionally smaller concentrations. 



As a result of such experiments Blagden formulated a law which states 

 that the depression of the freezing point of a dilute non-electrolyte is 

 proportional to the concentration of the solute. Raoult later found 



