92 FATS AND ALLIED SUBSTANCES [ch. 



(6) Add a little 1 o/o solution of osmic acid. A black colour is formed. (This re- 

 action is employed for the detection of fat in histological sections.) 



(c) Add to the oil a small piece of Alkanet (Anchusa ojicinalis) root, and warm 

 gently on a water-bath. The oil will be coloured red. Divide the oil into two portions 

 in test-tubes. To one add a little water, to the other alcohol. The coloured oil will 

 rise to the surface of the water in one case, and sink below the alcohol in the other. 

 The Alkanet pigment being insoluble in both water and alcohol, these liquids remain 

 uncoloured. 



Keep some of the linseed oil for Expt. 91. 



It is well known that the hydrocarbons of the unsaturated ethylene 



series CJi^n will combine directly with the halogens, chlorine, bromine 



and iodine to give additive compounds, thus: 



C2H4-|-Br2 = C2H4Br2 



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. 



FxpL 91, 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 : 



CnHgsCO-O— CHg 



I 

 C17H35CO • O— CH +3K0H =3Ci7H35COOK+CH20H • CHOH • CHgOH 



I glycerol 



CnHssCOO— CH2 



tristearin 



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

 is termed a soap. The ordinary soaps used for washing are mixtures 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. 



