158 NON-METALS AND THEIR COMBINATIONS. 



layer will be found 100 times that in the water layer. The process 

 of extraction must be repeated several times for the complete removal 

 of a substance from a liquid. 



Terms employed. The liquid in which a substance is dissolved 

 is called the solvent, while the substance, to avoid circumlocution, is 

 often called the solute. The word strength is frequently used to refer 

 to the amount of substance in solution, but a more exact term to 

 employ is concentration. A solution that contains a small quantity, 

 say 5 grammes of a substance in 100 c.c., is said to have a small con- 

 centration, or to be dilute. Concentrated solutions contain a relatively 

 large amount of dissolved substance ; they are often spoken of as very 

 strong solutions. Concentrating a solution is the removal of part of 

 the solvent by evaporation. A saturated solution is one that contains 

 the maximum amount of dissolved substance. This condition may 

 be attained by long agitation of the liquid with an excess of the sub- 

 stance. If the latter is a solid, it should be finely divided. The 

 solubility of a substance is its concentration in saturated solution, and 

 is expressed in terms of the number of grammes of the substance in 100 

 grammes of the solvent, or in 100 c.c. of the solution, or of the grammes 

 of solvent required to dissolve 1 gramme of the substance. The solu- 

 bility of substances varies with the temperature, being, as a rule, in 

 the case of solids, much greater at boiling than at ordinary temper- 

 ature. In most cases, when a hot concentrated solution is allowed 

 to cool, the excess of material over what corresponds to the solubility 

 at lower temperature, separates as crystals from the solution (see 

 Crystals, Chapter 1). But in some instances the excess of material 

 does not separate from the solution as it cools ; such a solution is 

 then said to be supersaturated. Crystallization can be induced by 

 placing in the cool solution a fragment of the same substance as that 

 in solution. A solution in contact with the solid substance cannot 

 be more than saturated with respect to that substance. There is an 

 equilibrium between the solid and the saturated solution. Sulphate, 

 thiosulphate, and chlorate of sodium have a marked tendency to give 

 supersaturated solutions, especially if the solutions are freed from all 

 floating particles by careful filtration. 



The solution of certain substances in water takes place with libera- 

 tion of heat and rise in temperature of the solution, while in other 

 cases there is an absorption of heat and a fall in temperature. Heat 

 of solution is the number of calories liberated or absorbed when the 

 weight of a substance in grammes corresponding to its chemical form- 

 ula (molecular weight) is dissolved in an unlimited amount of water. 



