66 PHYSIOLOGICAL CHEMISTRY 



The process is best carried out in alcoholic solution. Sodium 

 soaps are known as hard soaps; potassium soaps which are but- 

 tery in consistency are known as soft soaps. Calcium soaps 

 are very hard and insoluble. Soap is a useful cleansing agent. 

 Soiled articles, clothing, the hands, etc., usually are covered 

 with a layer of fatty material which entangles and holds parti- 

 cles of insoluble inorganic dirt. Soap emulsifies the fat and 

 the remaining material is carried away by the water or by the 

 lather, which takes up the particles of dirt mechanically. 



Since calcium soaps are very insoluble, hard water is not 

 good for washing purposes, as the calcium precipitates the soap 

 added, and thus interferes with its cleansing activities. 



Rancid Fats. Many natural fats, upon standing, acquire a 

 disagreeable taste and odor. This is due to the splitting of 

 some of the neutral fat into glycerine and fatty acids. The 

 lower fatty acids, such as those found in butter have a very 

 disagreeable taste and odor, hence the character of "rancid" 

 butter, etc. 



Detection and Identification. Acrolein Test. Fats are easily 

 detected by their physical properties, such as solubility, appear- 

 ance, greasiness, etc. A test given by all common fats is 

 known as the acrolein test. If a fat is heated to 300 it is de- 

 composed. The glycerine portion of the molecule loses water 

 and forms the unsaturated compound acrolein. The test is 

 obtained more readily if the fat is heated with a dehydrating 

 agent such as potassium acid sulphate, boric acid or phosphorus 

 pentachloride. Acrolein is easily recognized by its extremely 

 sharp and irritating odor. Since only substances containing glyc- 

 erine give the test, it may be used to distinguish between fats 

 and fatty acids or soaps. 



CH OH CH 2 



I II 



CHOH _2H 2 0-* CH 



I 

 H 2 OH CHO 



Acrolein. 



