120 SMITH'S INTERMEDIATE CHEMISTRY 



Definition of a Saturated Solution: A Warning. To 



avoid a common misconception, it must be noted that solution 

 is not a process of filling the pores of the liquid. If that were true, 

 approximately equal weights of all substances would find accommo- 

 dation in equal volumes of water. The fact is that, for example, 

 100 c.c. of water can dissolve 195 g. of silver fluoride, but only 

 0.00000035 g. of silver iodide, although the space available (if 

 there is any such space) is the same in both cases. 



The same conclusion is reached when we consider that two forms 

 of the same salt may have different solubilities. Thus, at 20, 

 Na2S0 4 ,10H 2 can give about 18 g. of NaaSC^ to 100 c.c. of water. 

 But anhydrous sodium sulphate Na2SC>4 at 20 gives 59 g. to the 

 same amount of water. 



The reader is also warned against the frequent definition of a 

 saturated solution as one containing all of the solute that it can 

 hold. A supersaturated solution evidently holds more. The 

 saturated solution under any given conditions is that solution 

 which, when placed in contact with excess of the solute, is found to 

 be in equilibrium. 



The molecular hypothesis may again be called to our assistance 

 in this connection. When we have a solute (either crystalline, or 

 liquid or gaseous) in contact with its saturated solution, and there- 

 fore in equilibrium with it, two opposing tendencies must balance 

 each other at the surface of contact. One of these is the tendency 

 of the solute particles to escape into solution, the other is the tend- 

 ency of the solute particles already in solution to return back to 

 the solute. The first of these tendencies (the intensity of the 

 hail of particles thrown off from the surface of the solute into a 

 given solvent, under given conditions of temperature and pressure) 

 we may regard as constant. The second tendency (the intensity 

 of the hail of particles returning from the solution to the surface 

 of the solute) will increase steadily as the concentration of the 

 solute particles in the solution increases. At one definite con- 

 centration only, therefore, can these two opposing tendencies 



