THE FATTY ACiDS 



99 



THE HIGHER FATTY ACIDS. 



The principal higher fatty acids are palmitic and stearic acids with 

 1 6 and 18 atoms of carbon in their molecules respectively. These 

 acids, together with oleic acid (p. 105), are obtained by the hydrolysis of 

 fats. The liquid oleic acid is removed by pressure, and the solid mix- 

 ture of palmitic and stearic acids, " stearine," is used for making candles. 



Properties. 



The higher fatty acids are white odourless solids. On heating, they 

 melt at a low temperature, and on further heating they boil giving oft 

 white vapours which condense on the cool parts of the test tube. 



They are insoluble in water, slightly soluble in alcohol and readily 

 soluble in ether. The solubility in alcohol may be seen by adding 

 some of the alcoholic solution to some alcohol containing a drop of 

 dilute caustic soda and a drop of phenolphthalein. The red colour of 

 the latter is discharged. 



They dissolve in dilute caustic alkali, aqueous or alcoholic, forming 

 solutions of soap. 



Soaps. 



Soaps are the sodium and potassium salts of the higher fatty acids ; 

 the former constitute hard soaps, the latter soft soaps. 



(1) Solutions of soap in water have an alkaline reaction to litmus 

 owing to partial hydrolysis of the salt. 



(2) On adding excess of mineral acid (H 2 SO 4 ) to a solution of soap 

 in water the fatty acids are liberated and form a precipitate which 

 floats to the surface. 



(3) On adding calcium chloride or magnesium sulphate to a solu- 

 tion of soap in water a curdy precipitate of the calcium or magnesium 

 salt is formed just as is obtained with hard water. 



(4) On adding finely powdered sodium chloride to a soap solution 

 the soap is salted out as a curdy mass which clings to the side of the 

 vessel. 



