OS MOLAR CONCENTRATIONS 41 



P varies as the number of molecules. De Vries (1884) fomid that 

 one ^ram-moleeule of sugar dissolved in water to make up a 

 litre, has at 0° C. an osmotie pressure of 22-4 atmos. (Practically 

 all gases at 0° C. and 760 nun. pressure ha^'e a gram molecular 

 volume of 22-4 litres, or conversely at 0° 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 (undissociated) dilute 

 solutions. Van't Hol'f (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. It follows that vapour pressure 

 varies directly with temperature. 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 this 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 

 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 coUigative 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. 303 lbs, 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. 



Class 1. O.P. Approximately 22-4 atmos., e.g. Sugar. 



„ 2. O.P. Decidedly greater than 22-4 ,, e.g. Salt. 

 „ 3. O.P. „ less „ 22-4 ,, e.g. Starch, 



The first class have been termed simple (undissociated) solutions. 



