SOLUTION. 161 



The above law, often called the law of Eaoult, does not hold in some cases, 

 especially in those of solutions of acids, bases, and salts in water. For ex- 

 ample, one molecular weight of sodium chloride, 58.06 grammes, dissolved in 

 1000 grammes of water, depresses the freezing-point about 3.5 C., or nearly 

 twice the amount produced by a cane-sugar solution of equivalent concentra- 

 tion, 1.89 C., which is taken as the molecular depression of normally acting 

 substances. It is also a noteworthy fact that aqueous solutions of substances 

 that have abnormal freezing-point depressions are just such as conduct an 

 electric current, while solutions of substances that give normal depressions do 

 not conduct a current. The same number of molecules of different substances 

 in a given amount of solvent produces the same lowering of the freezing-point, 

 and the molecular weights in grammes contain the same number of molecules. 

 The fact that the depression of the freezing-point of a solution of the molecular 

 weight in grammes of sodium chloride in 1000 grammes of water is much 

 greater than that of a similar solution of cane-sugar, can be accounted for on 

 the assumption that the number of particles in the sodium chloride solution 

 must be increased somehow over the number of particles in the sugar solution. 

 This increase can only take place by a decomposition of the molecule of sodium 

 chloride, thus, 



NaCl = Na + Cl. 



The particles Na and Cl must be different in condition from sodium and chlo- 

 rine as we know them in the free state, but as far as their effect upon the 

 freezing-point is concerned they act the same as undecomposed dissolved mole- 

 cules. This assumption of the decomposition of molecules is applied in the 

 case of all abnormally acting solutions where the freezing-point depressions 

 are greater than normal, and will be referred to again farther on under 

 lonization. 



In the field of medicine the determination of the freezing-point of certain 

 fluids is sometimes carried out in order to learn something of the manner in 

 which the organs are functioning. Normal blood has a lower freezing-point 

 than water, the difference is 0.56 C. A greater difference than this indicates 

 that the kidneys are not properly eliminating the solid waste products from 

 the blood. The freezing-point depression of normal urine is 1.2-2.3 C. Of 

 cows' milk it is 0.55-0.56 C. A lower depression indicates that the milk has 

 been tampered with. 



Boiling-points of solutions. These are always higher than the boiling- 

 points of the pure solvents. The boiling-point of any solution is that temper- 

 ature at which the vapor tension of the solution is equal to the atmospheric 

 pressure. Since the vapor tension of solutions is less than that of the pure 

 solvent, it follows that a solution must be heated to a higher temperature than 

 that at which the pure solvent boils, in order to make its vapor tension equal 

 to the atmospheric pressure ; in other words, to make it boil. The elevations in 

 the boiling-points are proportional to the concentrations of the different solutions of 

 any one substance. The molecular weight in grammes of normally acting sub- 

 stances, dissolved in 1000 grammes of water, elevates the boiling-point from 

 100 to 100.52 C. The difference, 0.52, is called the molecular boiling-point 

 constant for water. Just as in the case of freezing-points, so here a method 

 11 



