530 PEINCIPLES OF CHEMISTRY 



water but in alcohol, and even in ether. Dilute solutions of sodium 

 hydroxide produce a soapy feeling on the skin because the active base 

 of soap consists of caustic soda. 33 Strong solutions have a corroding 

 action. 



The chemical reactions of sodium hydroxide serve as a type for those 

 of a whole class of alkalis that is, of soluble basic hydroxides, MOIL 

 The solution of sodium hydroxide is a very caustic liquid that is to say, 

 it acts .in a destructive way on most substances, for instance on most 

 organic tissues hence caustic soda, like all soluble alkalis, is a poison- 

 ous substance ; acids, for example hydrochloric, serve as antidotes. 

 The action of caustic soda on bones, fat, starch, and similar vegetable 

 and animal substances explains its action on organisms. Thus bones, 

 when plunged into a weak solution of caustic soda, fall to powder, 34 



1,000 grams of a 5 p.c. solution occupies a volume of 946 c.c. , that is, less than the Water 

 serving to make the solution (see Note 18). 



53 Sodium hydroxide and some other alkalis are capable of hydrolysin.g saponifying, 

 as it is termed the compounds of acids with alcohols. If RHO (or R(HO) n ) represent the 

 composition of an alcohol that is, of the hydroxide of a hydrocarbon radicle and QHO 

 an acid, then the compound of the acid with the alcohol or ethereal salt of the given acid 

 will have the composition RQO. Ethereal salts, therefore, present a likeness to 

 metallic salts, just as alcohols resemble basic hydroxides. Sodium hydroxide acts on 

 ethereal salts in the same way that it acts on the majority of metallic salts namely, it 

 liberates alcohol, and forms the sodium salt of that acid which was in the ethereal ealt. 

 The reaction takes place in the following way -. 



RQO + NaHO = NaQO + RHO 

 Ethereal Caustic Sodium Ateohol 



salt soda salt 



Such a decomposition is termed saponification ; similar reactions were known very long 

 ago for the ethereal salts corresponding with glycerin, C3H 5 (OH) 5 (Chapter IX.), found 

 in animals and plants, and composing what are called fats or oils. Caustic soda, 

 acting on fat and oil, forms glycerin, and sodium salts of those acids which were in 

 union with the glycerin in the fat, as Chevreul showed at the beginning of this century. 

 The sodium salts of the fatty acids are commonly known as soaps. That is to say, 

 soap is made from fat and caustic soda, glycerin being separated and a sodium salt or 

 soap formed. As glycerin is usually found in union with certain acids, so also are the 

 sodium salts of the same acids found in soap. The greater part of the acids found in 

 conjunction with glycerin in fats are the solid palmitic and stearic acids, C 16 H 32 O2 and 

 CigHjaOjj, and the liquid oleic acid, Ci^EL^Oz- In preparing soap the fatty substances 

 are mixed with a solution of caustic soda until an emulsion is formed ; the proper 

 quantity of caustic soda is then added in order to produce saponification on heating, 

 the soap being separated from the solution either by means of an excess of caustic soda 

 or else by common salt, which displaces the soap from the aqueous solution (salt water 

 does not dissolve soap, neither does it form a lather). Water acting on soap partly 

 decomposes it (because the acids of the soap are feeble), and the alkali set free acts 

 during the application of soap. Hence it may be replaced by a very feeble alkali. 

 Strong solutions of alkali corrode the skin and tissues. They are not formed from soap, 

 because the reaction is reversible, and the alkali is only set free by the excess of water 

 Thus we see how the teaching of Berthollet renders it possible to understand many 

 phenomena which occur in every -day experience (see Chapter IX., Note 16). 



54 Ob this is founded the process of Henkoff and Engelhardt for treating bonea 



