WYCKOFF: FORCES BETWEEN ATOMS IN SOLIDS 589 



It seems possible to state as a general rule that strong sol- 

 vents (i. e., those possessed of large external forces) dissolve 

 strong solutes, while weak solvents dissolve weak solutes. Weak 

 solutes are not usually markedly soluble in strong solvents nor 

 is the reverse case true. Water dissolves electrolytes; carbon 

 disulfide and benzene dissolve organic compounds; but electro- 

 lytes are not appreciably soluble in carbon disulfide or benzene, 

 while water dissolves only those organic compounds which are 

 possessed of large fields — the acids, alcohols, sugars and the like. 



Solubility results, in fact, from a chemical reaction between 

 solvent and solute. In order that marked solution can take 

 place, it is necessary that the stray fields about the solvent and 

 the dissolving substance shall each be strong enough to "open 

 up" and make reactive the condensed systems of the other. A 

 certain solubility can be considered to exist in all cases unless 

 the temperature is carried too low. The ordinary tendency for 

 the solute to "vaporize" into the liquid will be enhanced to an 

 extent depending upon the added stray fields of the solvent. 



The solubility of molecular compounds in non-associated 

 liquids is perfectly straightforward. There will always be some 

 solubility, increasing in amount as the solvent fields, which in 

 such liquids are already pretty well opened up, become of in- 

 creasing importance compared with those of the solute. In 

 order that a substance may be strongly soluble in an associating 

 liquid, it must possess fields great enough more or less to break 

 up this association. Two types of solids have strong outside 

 fields: valency compounds and electrolytes. The fields upon 

 the surface of valency Solids are due to the tendency of the 

 surface atoms to complete their clusters of eight electrons. Ex- 

 cept directly upon the surface, a valency compound is very 

 thoroughly saturated. The surface atoms will be able to com- 

 bine with the solvent molecules, but little action will result be- 

 cause of the much greater force within the solid material. Ad- 

 sorption, rather than solution, results from this reaction. Elec- 

 trolytes possess doublets of large moments which are able to 

 open up to a large degree the fields of the solvent. The extent 

 of the solution of an electrolyte depends upon 



