442 SMITH'S INTERMEDIATE CHEMISTRY 



separated, if the water surrounding the shell is replaced by pure 

 water at intervals until all the non-colloid has be'en removed. 

 This method of separation is called dialysis. 



Finally, matter in colloidal suspension may be coagulated (or 

 flocculated) by addition of electrolytes or other colloids, and is 

 then precipitated. 



Colloidal Matter in Soap Solutions. Explanation of 

 Salting Out. Soaps, being salts of weak acids, are some- 

 what hydrolyzed in solution. Letting R stand for the hydrocar- 

 bon part of the acid radical: 



Na(C0 2 R) + H 2 <= H(CO 2 R) + NaOH. 



The free acid HC0 2 R thus produced combines with the original 

 salt NaC0 2 R to form an acid salt NaH(C0 2 R) 2 . This acid salt 

 is a colloidal substance, and exists in colloidal suspension in the 

 soap solution, in equilibrium with the ions and molecules of the 

 original salt and the NaOH. 



The capacity for being coagulated and precipitated, which is 

 characteristic of colloidal matter, is shown very clearly by soap 

 solutions. Most sodium salts will coagulate a soap solution and 

 precipitate the soap as a curd. The acid salt NaH(C0 2 R) 2 seems 

 to collect (adsorb) and carry down with it the most of the sodium 

 hydroxide. As both of the substances on the right of the equa- 

 tion (above) are thus precipitated, the equilibrium is displaced 

 to the right, and the precipitation becomes complete. This 

 explains the process of " salting out " (p. 439) which plays so large 

 a part in the manufacture of soap. 



Causes of the Cleansing Action of Soap. The chief use of 

 soap solution is in removing grease and dirt from yarn, cloth, or 

 clothing, and from woodwork and kitchen utensils. Soap solu- 

 tion has two more or less distinct properties, one of which enables 

 it to remove oil or grease (viscous, insoluble liquids), and the 



