CLASSIFICATION OF SOLUTIONS 45 



at this concentration. The degree of ionisation may be deter- 

 mined by estimating the amount of resistance of the solution to 

 a small electric current (conductivity method), or it may be 

 approximately calculated from the lowering of the freezing point. 

 (For univalent strong electrolytes at concentrations not exceed- 

 ing 0-1 molar, Noyes and Falk consider that the percentage error 

 is in most cases between 1 and 4.) 



One may note in passing that the ions of many electrolytes 

 possess the property of uniting with other ions, or even with- 

 non-electrolytes in solution, to form complex ions. For ex- 

 ample, ions cannot normally remain free in aqueous solution, 

 but become hydrated. A hydrated ion is sometimes termed an 

 ionic micelle. 



In solutions of the third class, the O.P. and other colligative 

 properties point to a reduction in the number of particles in 

 solution. A clubbing of molecules has taken place. Because 

 the substances that compose this group have a somewhat gluey 

 consistency, Graham called them colloids (Gr. KoXXtj =glue). The 

 physics of colloidal complexes will be dealt with in a separate 

 chapter. Here we merely wish to draw attention to their low 

 osmotic pressure. Colloid substances may be converted into non- 

 colloid or crystalloid derivatives, and so liberate energy, e.g. 

 starch, a colloid having practically no O.P. may be readily hydro- 

 lysed to maltose, which is a crystalloid non-electrolyte, having 

 a molal O.P. Glucose, which has a molal O.P., may be stored 

 in the liver as glycogen a colloid, which again readily undergoes 

 hydrolysis to glucose. (See Chap. VIII., Colloids.) 



