REACTIONS IN HOMOGENEOUS SYSTEMS 29 



produces the same result as diffusion, namely that after all 

 the solvent has passed through the membrane to dilute the 

 solution there is one solution of uniform concentration. This 

 process is called osmosis. 



DIALYSIS 



A mixture of substances, some of which pass through the 

 membrane and others which do not, can be separated by 

 means of diffusion through a membrane. This process is 

 known as dialysis and it is made use of to separate diffusible 

 from non-diffusible substances.* 



OSMOTIC PRESSURE 



If the solution inside the semi-permeable membrane is 

 enclosed in rigid walls the passage of the solvent inwards 

 will produce a pressure. As the pressure inside increases the 

 solvent is prevented from entering. When the pressure 

 inside the membrane balances the osmosis a steady pressure is 

 maintained which is called the osmotic pressure of the solution. 



VAPOUR PRESSURE. 



As it is not an easy matter to obtain good semi-permeable 

 membranes the osmotic pressure is usually measured by some 

 indirect method. These indirect methods depend on the 

 decrease of the vapour pressure of the solvent by the dissolved 

 substance and we shall now study the reason why the decrease 

 of the vapour pressure is proportional to the osmotic pressure 

 and finally how to measure it by freezing point or boiling point 

 determinations. 



In order to understand the relation of these indirect methods 

 to the direct measurement of osmotic pressure, it is necessary 

 to refer to the kinetic theory of gases. The gas molecules 

 are conceived as being in constant oscillating motion and 

 collisions occur between the various molecules and between 

 the molecules of the gas and the walls of the containing vessel. 

 The pressure of the gas is dependent on the. number of mole- 

 cules striking the wall at any one time. The number of 

 collisions with the wall depends on the rate of movement and 

 the number of molecules in a given volume. The rate of 

 movement is a function of the temperature. Hence equal 

 volumes of different gases at the same temperature and pressure 

 contain the same number of molecules. f 



* T. Graham, Phil. Trans., 1861, vol. 157, p. 186. 



f The mean kinetic energy of the various kinds of molecules is the 

 same. For further information on this Law of Avogadro see text- 

 books of physical chemistry. 



