160 NON-METALS AND THEIR COMBINATIONS. 



Henry's law holds in the case of absorption of gases by saline solutions, if 

 the gas has no chemical action on the salt in solution, for example, in the 

 absorption of carbon dioxide, oxygen, or nitrogen, by a solution of sodium 

 chloride (common salt). When the gas acts chemically on the dissolved salt, 

 as carbon dioxide does on ordinary sodium phosphate or sodium carbonate, 

 one portion of the gas is absorbed in accordance with Henry's law, and an 

 additional portion is absorbed as a result of chemical action and is independent 

 of pressure. 



Freezing-points of solutions. The freezing-point of a solution is always 

 lower than that of the pure solvent. It is also easily observed, by introducing 

 small quantities of solutions and pure solvents over the mercury in barometer 

 tubes, that the vapor tension of the solutions is always less than that of the 

 pure solvents, no matter what the temperature is. This difference in vapor 

 tension accounts for the fact that the freezing-point of solutions is depressed. 

 Theoretical considerations show that freezing (separation of some of the solvent 

 in the solid state) can take place only at such a temperature at which the solu- 

 tion and the solid state of the solvent have the same vapor tension, whereby 

 they are in physical equilibrium and can co-exist permanently (see discussion 

 under Efflorescence and Deliquescence, p. 152). Since the vapor tension of a 

 solution is always less than that of the pure solvent, it follows that the freezing- 

 point of a solution must be lower than that of the pure solvent, in order that 

 the vapor tension of the solution and of the ice that separates may balance 

 each other. (On this principle the low temperature of freezing mixtures, such 

 as snow (or ice) and salt, is explained). 



For a description of apparatus and details of method in making determina- 

 tions of the freezing-points of solutions, the student must be referred to books 

 on physical chemistry. 



For solutions not too concentrated and in which there is no chemical action 

 between the solvent and the substance dissolved, the following law has been 

 found to hold : The depression of the freezing-point is directly proportional to the 

 iveight of dissolved substance in a given amount of the solvent. By calculations 

 made on the results obtained with dilute solutions, the following law has been 

 found to hold theoretically for molecular quantities of substances : The molec- 

 ular weights in grammes of different substances dissolved in 1000 grammes of the 

 same solvent produce the same depression of the freeezing -point. The depression 

 thus produced is called the molecular depression constant, and has a different 

 numerical value for each solvent. For water it is 1.89 C.; for benzene, 4.9 C., 

 and for phenol, 7.5 C. 



The above law gives a basis for a method of determining molecular weights, 

 which was first applied extensively by Raoult, and is sometimes called the 

 cryoscopic method. It is valuable in the case of substances which cannot be 

 volatilized without decomposition. The molecular weight is calculated from 

 the equation 



D = d X Vv-' 



M X g 



in which D is the amount of depression in any actual experiment, d the molec- 

 ular depression constant, W the weight of the substance, M its molecular 

 weight, and g the weight of the solvent in grammes. 



