Solution of Solids. 3 ( J 



When a lump of salt is dropped into water its surface 

 molecules are drawn outward by the surrounding water so 

 that the effective pull upon them toward the center is made 

 less by it. It is therefore easier for a given temperature 

 to throw out into the water some of the molecules in the 

 surface layer of the salt. Stated in another way the water 

 surrounding the salt so weakens the surface tension of the 

 solid lump of salt that solution takes place at a lower tem- 

 perature. 



53. Influence of Temperature on Solution. Since it is the 

 absorption of heat which causes solution it is clear that the 

 higher the temperature the more rapid will the solution be. 

 It is even true that any solid will evaporate or dissolve if 

 only it is given a high enough temperature, provided its 

 molecules are not decomposed at a lower temperature than 

 that which is required to overcome the force of cohesion 

 which makes them solid. It is a matter of common ob- 

 servation that substances dissolve more rapidly in warm 

 than in cold water and it is equally true that the soluble 

 salts in the soil will form more rapidly when the soil is 

 warm than when it is cold and it is because of this fact, in 

 part, that crops grow better under the higher temperature. 



54. A Saturated Solution. When conditions are favor- 

 able for the solution of a solid in water there comes a time 

 when there is no increase in the concentration of the solu- 

 tion. A condition is reached which is analogous to the 

 air saturated with moisture, when as many molecules pass 

 from the solution and become fixed upon the face of the 

 solid as are thrown by heat from the face of the solid into 

 the solution. When a solution reaches this condition it is 

 said to be saturated. 



In the case of the soil water, where the roots of plants 

 are brought in contact with it, if the roqts are removing 

 the materials which are dissolved, their action hastens the 

 rate of solution, for they prevent it from becoming satur- 

 ated and thus prevent the return to the soil grains of par- 

 ticles once removed. 



