532 PRINCIPLES OF CHEMISTRY 



ing with the alkali, then, naturally, a salt of the corresponding acid is 

 formed. For example, chlorine and sulphur act in this way 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 forms the hypochlorite, NaCIO In the same way, 

 by the action of sodium hydroxide on sulphur, hydrogen sulphide is 

 formed, which acts on the soda forming sodium sulphide, in addition to 

 which sodium thiosulphate is formed (see Chapter XX.) By virtue of 

 such reactions, sodium hydroxide acts on many metals and non-metals. 

 Such action is often accelerated by the presence of the oxygen of the 

 air, as by this means the formation of acids and oxides rich in oxygen 

 is facilitated. Thus many metals and their lower oxides, in the presence 

 of an alkali, absorb oxygen and form acids. Even manganese peroxide, 

 when mixed with caustic soda, is capable of absorbing the oxygen of 

 the air, and forming sodium manganate. Organic acids when heated 

 with caustic soda give up to it the elements of carbonic anhydride, 

 forming sodium carbonate, and separating that hydrocarbon group 

 which exists, in combination with carbonic anhydride, in the organic 

 acid. 



Thus sodium hydroxide, like the soluble alkalis in general, ranks 

 amongst the most active substances in the chemical sense of the term, 

 and but few substances are capable of resisting it. Even siliceous rocks, 

 as we shall see further on, are transformed by it, forming when fused 

 with it vitreous slags. Sodium hydroxide (like ammonium and potassium 

 hydroxides), as a typical example of the basic hydrates, in distinction, 

 from many other basic oxides, easily forms acid salts with acids (for 

 instance, NaHSO 4 , NaHCO 3 ), and does not form any basic salts at 

 all ; whilst many less energetic bases, such as the oxides of copper 

 and lead, easily form basic salts, but acid salts only with difficulty. 

 This capability of forming acid salts, particularly vith polybasic 

 acids, may be explained by the energetic basic properties of sodium 

 hydroxide, contrasted with the small development of these properties 

 in the bases which easily form basic salts. An energetic base is 

 capable of retaining a considerable quantity of acid, which a slightly 

 energetic base would not have the power of doing* Also, as will be 

 shown in the subsequent chapters, sodium belongs to the univalent 

 metals, being exchangeable for hydrogen atom for atom-^-that is, 

 amongst metals sodium may, like chlorine amongst the non-metals, serve 

 as the representative of the univalent properties. Most of the elements 

 which are not capable of forming acid salts are bivalent. Whence 

 it may be understood that in a bibasic acid for instance, carbonic, 



