BIOLOGICAL CHEMISTRY 



The process of diffusion depends on the collisions of the 

 molecules, and as the number of collisions depends on the con- 

 centration of the solution the rate of diffusion is proportional 

 to the differences of concentration. 



Evaporation is produced by the movement of some of the 

 molecules out of a fluid into the vapour space above. When 

 the vapour space becomes saturated, i.e. the same number of 

 molecules pass from the vapour space to the solution as pass 



76Qmn. 



. -1,86 IOOioc-5 



FIG. 5. Diagram illustrating the relation of concentration (osmotic 

 pressure) to vapour pressure, depression of freezing point and rise 

 of boiling point. 



The upper curve represents the pure solvent and the lower one the solution. The line 

 across the diagram shows the atmospheric pressure of 760 mm. of mercury. When the vapour 

 pressure reaches this pressure the solution boils. The actual differences are exaggerated, 

 but the real points are freezing point of water at o* C. and 4-58 mm. Hg. pressure. A molecular 

 solution causes a depression of r86 C. in the freezing point and a rise of 0-515 C. in the 

 bi til ing point. 



from the solution to the vapour space, a steady condition 

 results, which is called the vapour pressure of the solution. 

 The vapour pressure of a solution depends, like the gas pres- 

 sure, .on the temperature and the number of molecules in a 

 given space. In a solution the molecules of the solvent are 

 fewer per unit space because the solute molecules lie between 

 them, hence the vapour pressure of a solvent is decreased 

 by dissolving something in it. 



