COLLIGATIVE PROPERTIES OF A SOLUTION 43 



molecular volume of 22-4 litres, or conversely at C. it would 

 require a pressure of 22-4 atmos. to reduce a gram molecular 

 volume to one litre.) De Vries, Pfeffer, and others have shown 

 that this is true not only for sugar, but for all simple (undis- 

 sociated) solutions. Van't Hoff (1887) pointed out that the 

 osmotic pressure of simple solutions is the same quantitatively 

 as gas pressure. We have already pointed out that vapour 

 pressure is a function of molecular activity, and may be taken 

 as an index of the kinetic energy of the liquid. The kinetic energy 

 again varies with the temperature of the liquid system. It 

 follows that vapour pressure varies directly with temperature. We 

 have seen that the putting of a substance into solution lowers 

 the vapour pressure of the solvent and, therefore, lowers its heat 

 content. This can be deduced from the second law of energetics. 

 From what has been said regarding the lowering of vapour 

 pressure, which always occurs when one substance is dissolved 

 in another, it may be inferred that the boiling point of a liquid 

 is always raised when a substance is dissolved in it. (These only 

 apply to instances where the V.P. of the solute is negligible.) A 

 very volatile substance, ether, for instance, would raise the V.P. 

 and lower the B.P. 



Correlated with these two sequelae of solution is the lowering 

 of the freezing point. (Part II.) 



The magnitudes of the osmotic pressure, lowering of the vapour 

 pressure and freezing point, and raising of the boiling point 



n 

 depend in general on -^, the number of particles in solution per 



unit volume. Because these magnitudes are all interrelated and 

 are interdependent they have been named the colligative properties 

 of a solution. They are definite physical quantities quite indepen- 

 dent of semi-permeable membranes, etc. The membranes make 

 osmotic pressure apparent. 



Osmotic pressure is of considerable magnitude. We have seen 

 that a gram-molecular solution has an osmotic pressure of 

 22-4 atmos., i.e. 330 Ibs. per sq. inch. The ordinary dilute 

 laboratory reagents develop a pressure of about 50 atmos. In 

 a plant, root pressure has been estimated at about 60 feet of 

 water. 



If, however, one were to tabulate the osmotic pressure of 

 gram-molecular solutions of all substances, one would find that 

 solutions could be divided into three great classes according to 

 pressure. 



