82 THE FATS AND LIPASES [CH. 



It is well known that the hydrocarbons of the unsaturated ethylene 

 series CnH^ will combine directly with the halogens, chlorine, bromine 

 and iodine to give additive compounds, thus : 



ethylene bromide 



The acids of this series also behave in the same way, and since many 

 plant fats contain members of the series, the fats will also combine with 

 the halogens. 



Expt. 83. To show the presence of unsaturated groups in a fat. To a little of the 

 linseed extract add bromine water. Note the disappearance of the bromine and the 

 formation of a solid product. 



One of the most important chemical reactions of fats is that known 

 as saponification. When a fat is heated with an alkaline hydroxide the 

 following reaction takes place : 

 C 17 H 36 CO-0 CH 2 



C 17 H3sCO O CH + 3KOH = 3C 17 H35COOK + CH 2 OH CHOH CH 2 OH 



glycerol 



C^H-tfCO'O CH 2 

 tristeariri 



The potassium salt, potassium stearate, of the fatty acid, stearic 

 acid, is termed a soap. The ordinary soaps used for washing are mix- 

 tures of such alkali salts of the various fatty acids occurring in vegetable 

 and animal fats, and are manufactured on a large scale by saponifying 

 fats with alkali. The soaps are soluble in water, so that when a fat is 

 heated with a solution of caustic alkali, the final product is a solution of 

 soap, glycerol and excess of alkali. The soap is insoluble in saturated 

 salt (sodium chloride) solution, and when such a solution is added to 

 the saponified mixture, the soap separates out and rises to the 

 surface of the liquid. This process is known as " salting out." If the 

 saponified mixture is allowed to cool without salting out, it sets to 

 a jelly-like substance. When caustic potash is used for saponification 

 and the product is allowed to set, a " soft " soap is formed. Hard soaps 

 are prepared by using caustic soda and salting out. 



The properties of soaps in solution are important. When a soap 

 goes into solution, hydrolysis takes place to a certain extent with the 

 formation of free fatty acid and free alkali. The free fatty acid then 

 forms an acid salt with the unhydrolyzed soap. This acid salt gives rise 

 to an opalescent solution and lowers the surface tension of the water 

 with the result that a lather is readily formed. * 



The property of soaps of lowering surface tension is the reason for 



