408 



SMITH'S INTERMEDIATE CHEMISTRY 



Explanation of Osmotic Pressure. For a complete dis- 

 cussion of osmotic pressure, the reader is referred to a modern 

 text-book of physical chemistry. A brief explanation by means 

 of the molecular hypothesis, however, may be given here. 



The molecules of water in the pure water outside the cell, and 

 the molecules of water and of sugar in the solution inside the cell, 

 are all in rapid motion (p. 94). When, in consequence of this 

 motion, they strike the membrane, water molecules have a chance 

 of passing through, but sugar molecules are all turned back. 

 Now the concentration of water molecules in the pure water out- 

 side, striking the membrane and attempting to enter the cell, is 

 greater than the concentration of water molecules in the solution 

 inside, striking the membrane and attempting to leave the cell 

 (compare vapor pressures, p. 117). Hence more water molecules 

 will be entering than leaving, and the level of the liquid inside the 

 tube must rise in consequence. 



Why does the level of the liquid stop rising when a definite 

 hydrostatic head has been established? Because now, although 

 there is still a greater concentration of water molecules in the 

 pure water outside than in the solution inside, water molecules 

 attempting to enter the cell through the membrane are opposed 

 by the hydrostatic pressure, while water molecules attempting 

 to leave are assisted in their passage. We have on one side of the 

 membrane more water molecules with a smaller chance of getting 

 through, on the other side fewer water molecules with a greater 

 chance of getting through. Equilibrium is reached, evidently, 

 when these two factors counterbalance. 



The student should note very carefully the fact that the sugar 

 molecules are not directly concerned in the phenomenon of osmotic 

 pressure. Their function is merely to reduce the concentration 

 of water molecules in the solution inside the cell. Any solute, with 

 respect to which the membrane is similarly impermeable, will 

 give the same effect, the osmotic pressure at any given tempera- 

 ture being dependent only upon the fraction of solute molecules in 



