SOLUTION 121 



counterbalance, namely that of the saturated solution. The rate 

 at which solute particles are returning from the solution just 

 equals, in this case, the rate at which they are entering. With 

 unsaturated solutions, containing less solute, the number of re- 

 turning particles will be deficient, and the solute will continue to 

 dissolve until it all disappears or saturation is reached. With 

 supersaturated solutions, on the other hand, containing more 

 solute, the number returning will be in excess, and deposition of 

 home-coming particles on the solute surface will continue until 

 this excess is wiped out. 



If, however, we have a supersaturated solution in which no 

 free solute is present, the solute particles in the solution have no 

 home to return to, no surface upon which they can deposit them- 

 selves. They are therefore compelled to continue wandering 

 around and around in the solution, having lost their equilibrium 

 completely. By violent shaking or stirring we may succeed in 

 inducing crystallization in such a solution, but the only certain 

 means of establishing equilibrium conditions is inoculation with 

 a small fragment of the solute. 



Units Used in Expressing Concentrations. The concen- 

 trations of solutions, saturated and otherwise, are sometimes 

 expressed in physical, and sometimes in chemical, units of weight. 

 When physical units are employed, we give the number of grams 

 of the solute held in solution by one hundred grams of the solvent. 



When chemical units of weight are employed, two different plans 

 are possible, and both are in use. Either the equivalent (p. 53) or 

 the atomic weights may be taken as a basis of measurement. In 

 the former case, the solutions are called normal solutions, and in 

 the latter, molar solutions. 



A nonoal sojution contains one gram-equivalent of the solute 

 in one liter of solution (not in 1 1. of solvent). The word " equiva- 

 lent " has been used hitherto only of elements, and this application 

 of the expression involves an extension of its meaning. An 



