14 FATS, OILS, AND WAXES 



which, however, in this case, combines with the alkali to form 



a salt : 



C 17 H 35 COOCH 2 CH 2 OH 



C 17 H 35 COOCH + 3KOH = 3 C 17 H 35 COOK + CHOH 



C 17 H 35 COOCH 2 Potassium stearate, CH 2 OH 



Tristearin, a fat a soap Glycerol 



It so happens that the sodium and potassium salts of 

 palmitic, stearic, and oleic acids dissolve in water forming 

 opalescent alkaline solutions which readily give a lather, and 

 can, therefore, be used as soaps,* and hence the process by which 

 they are made from fats is called saponification. Although 

 alkali metal salts of other organic acids do not exhibit the 

 characteristics of soap, the term saponification has nevertheless 

 been extended to include all cases of the decomposition of an 

 ester into the corresponding 'alcohol and the salt of the acid, 



* The sodium and potassium salts of oleic acid and of the higher fatty acids, 

 such as palmitic and stearic acids, when dissolved in water, are, to a large extent, 

 hydrolysed into free fatty acid and caustic soda according to the equation : 



C 17 H 35 COONa + H 2 O = C 17 H 35 COOH + NaOH 

 Sodium stearate Stearic acid 



The stearic acid combines with some of the unhydrolysed soap to form an 

 insoluble acid salt, giving rise to an opalescent or turbid solution. It is this in- 

 soluble acid salt which is responsible for the formation of a lather on shaking such 

 a solution. The detergent or cleansing action of soap is dependent on the above 

 reaction since the caustic soda detaches the greasy dirt which then becomes 

 enveloped in a layer of soap solution from the lather and is so carried away. 



In this connexion it is interesting to note the similar effect of soap on the 

 formation of emulsions. 



An emulsion may be defined as a mixture, under special conditions, of two 

 otherwise immiscible liquids. Thus, for example, if olive oil is shaken up with 

 water, the two liquids rapidly separate as soon as the shaking ceases. If, how- 

 ever, a little soap solution or some other substance such as gum acacia, traga- 

 canth, saponin (see p. 181), or white of egg be added and the shaking repeated, 

 an emulsion results owing to the oil particles being enveloped in a layer of soap 

 or other substance which prevents their coalescing. Milk is an example of a 

 naturally occurring emulsion ; so also is latex contained in plants. 



If pure olive oil, free from oleic or othe? acid, is shaken up with caustic 

 soda no emulsion is produced ; on the other hand, olive oil which has been kept 

 some time and contains free oleic acid, when shaken up with caustic soda does 

 produce an emulsion, thus showing that the emulsifying agent is not the free 

 alkali but the soap produced in the second case from the soda and the oleic acid. 



This may be also illustrated by Biitschli's experiment which consists in 

 placing a drop of old olive oil containing 9 per cent of oleic acid on a little 

 0-06 per cent aqueous solution of sodium carbonate. If examined under the 

 microscope it will be seen to consist of a fine honeycomb structure, consisting of 

 particles of oil, the whole apparently exhibiting amreboid movements ; these 

 latter are due to difference in surface tension. 



