SODIUM 523 



saponified with a solution of caustic soda that is to say, they form 

 with it Koaps soluble in water, or sodium salts of the organic acids 

 contained in the fats. 35 The clearest reactions of sodium hydroxide 

 are determined by the fact that it saturates all acids forming salts with 

 them, which are almost all soluble in water, and in this respect caustic 

 soda is as characteristic amongst the bases as nitric acid is among the 

 acids. It is impossible to detect sodium by means of the formation of 

 precipitates of insoluble sodium salts, as may be done with other 

 metals, many of whose salts are but slightly soluble. The powerful 

 alkaline properties of caustic soda determine its capacity for com- 

 bining with even the feeblest acids, its property of disengaging 

 ammonia from ammonium salts, its faculty of acting on salts whose- 

 bases are insoluble in water, <fcc. If a solution of the salt of almost 

 any metal be mixed with caustic soda, then a soluble sodium salt will 

 be formed, and an insoluble hydroxide of the metal will be separated 

 for instance, copper nitrate yields copper hydroxide, Cu(NO 3 ) 2 

 + L ) XaHO=Cu(HO). 2 + 2]SraNO3. Even many basic oxides as, for in- 

 stance, the oxides of zinc and aluminium precipitated by caustic soda, 

 are capable of combining with it and forming soluble compounds, and 

 therefore caustic soda in the salts of such metals first forms a pre- 

 cipitate of hydroxide, and then, employed in excess, dissolves this, 

 precipitate. This phenomenon occurs, for instance, when caustic soda 

 is added to the salts of aluminium. This shows the property of such 

 an alkali as caustic soda of combining not only with acids, but also- 

 with feeble basic oxides. For this reason caustic soda acts on most 

 elements which are capable of forming acids or oxides similar to- 

 them ; thus, for instance, the metal aluminium gives hydrogen with 

 caustic soda in consequence of the formation of alumina, which com- 

 bines with the caustic soda that is, in this case, the caustic alkali 

 acts on the metal just like an acid. If the substance which is mixed 

 with the caustic soda is capable of combining with the hydrogen 

 evolved (aluminium does not form such a compound), then such a, 

 compound will be formed. Thus, for instance, in this way phosphorus 

 acts on caustic soda, yielding hydrogen phosphide. If the hydrogen 

 compound disengaged is capable of combining with the alkali, then, 

 naturally, a salt of the corresponding acid is formed. For instance, 

 chlorine and sulphur act thus on caustic soda. Chlorine, with the 

 hydrogen of the caustic soda, forms hydrochloric acid, and the latter 

 forms common salt with the sodium hydroxide, whilst the other atom 

 in the molecule of chlorine, C1. 2 , takes the place of the hydrogen, and 



33 As explained in Note 83. 



