10 THE PROPERTIES OF SOLUTIONS. [CH. I. 



positive charge in acid solution and a negative charge in 

 alkaline solution. At some particular reaction they seem 

 to be electrically neutral, i.e. kataphoresis cannot be 

 observed. This reaction is known as " the iso-electric 

 point " of the particular colloid. It is discussed in more 

 detail on p. 31. 



F. The precipitation of colloids. 



Colloids, as we have seen, are two phase solutions. 

 One, the solid, phase contains a high concentration of the 

 solute and a low concentration of the solvent : the other, 

 the liquid, phase contains a low concentration of the 

 solute in the solvent. By certain changes in the condi- 

 tions the solid phase can be dehydrated, so that the solution 

 may become opalescent. An increase of this effect may 

 result in the formation of particles visible to the naked 

 eye, or even a dense precipitate that can, in some cases, 

 be removed completely by filtration. In some cases this 

 precipitate can be " dissolved " or " dispersed " by revert- 

 ing to the original conditions. In other cases the change is 

 irreversible, the material having been " coagulated." 



It is impossible to discuss fully the various conditions 

 that tend to cause aggregation (i.e. precipitation) on the 

 one hand, or dispersion (i.e. solution) on the other, since 

 they vary considerably with different colloids. But it is 

 important to note that many cases can be explained fairly 

 satisfactorily on the theory that the dispersed or dissolved 

 condition of a colloid is due to the fact that it carries an 

 electric charge, the removal of which causes precipitation. 

 Some examples of this are given below : 



(a) By colloids with an opposite electrical charge. 



If ferric chloride be thoroughly dialysed a colloidal 

 suspension of ferric hydroxide is obtained, generally known 

 as " dialysed iron." This carries a positive charge. If 

 this be added to certain albumins which carry a negative 

 charge the two colloids mutually precipitate one another. 

 This gives us a valuable method for removing certain 

 proteins from solution. (See Ex. 310.) 



