THE HALOGENS 479 



powder (they are not obtained free from metallic chlorides), must be 

 counted as salts, because their metals are capable of substitution. But 

 the hydrate HC1O corresponding with these salts, or hypochlorous 

 acid, is not obtained in a free or pure state, for two reasons : in the 

 first place, because this hydrate, as a very feeble acid, splits up (like 

 H 2 CO 3 or HNO 3 ) into wajer and the anhydride, or chlorine monoxide. 

 C1 2 O = 2HC1O H 2 ; and, in the second place, because, in a number 

 of instances, it evolves oxygen with great facility, forming hydrochloric 

 acid : HC1O = HC1 + O. Both hypochlorous acid and chlorine 

 monoxide may be regarded as products of the metalepsis of water, 

 because HOH corresponds with C1OH and. C1OC1. Hence in many 

 instances bleaching salts (a mixture of hypochlorites and chlorides) 

 break up, with the evolution of (1) chlorine, under the action of an 

 excess of a powerful acid capable of evolving hydrochloric acid from 

 sodium or calcium chlorides, and this takes place most simply under 

 the action of hydrochloric acid itself, because (p. 462) NaCl + NaCIO 

 + 3HC1 = 2NaCl + HC1 + C1 2 + H 2 O ; (2) oxygen, as we saw in 

 Chapter III. The bleaching properties and, in general, oxidising action 

 of bleaching salts is based on this evolution of oxygen (or chlorine) > 

 oxygen is also disengaged on heating the dry salts for instance, 

 NaCl 4- NaCIO = 2NaCl + O ; (3) and, lastly, chlorine monoxide, 

 which contains both chlorine and oxygen. Thus, if a little sulphuric, 

 nitric, or similar acid (not enough to liberate hydrochloric acid 

 from the Ca01 2 ) be added to a solution of a bleaching salt (which 

 has an alkaline reaction, owing either to an excess of alkali or 

 to the feeble acid properties of HC10), then the hypochlorous acid set 

 free gives water and chlorine monoxide. If carbonic anhydride (or 

 boracic or a similar very feeble acid) act on the solution of a bleaching 

 salt, then hydrochloric acid is not evolved from the sodium or calcium 

 chlorides, but the hypochlorous acid is displaced and gives chlorine 

 monoxide, 31 because hypochlorous acid is one of the most feeble acids. 

 Another method for the preparation of chlorine monoxide is based 

 on these, feeble acid properties of hypochlorous acid. Zinc oxide and 

 mercury oxide, under the action of chlorine in the presence of water, 

 do not give a salt of hypochlorous acid, but form a chloride and 

 hypochlorous acid, which fact shows the incapacity of this acid to 



31 For this reason it is necessary that in the preparation of bleaching powder the chlorine 

 should be free from hydrochloric acid, and even the lime from calcium chloride. An 

 excess of chlorine, in acting on a solution of bleaching powder, may also give cMorine 

 monoxide, because calcium carbonate also gives chlorine monoxide under the actioir of 

 chlorine. This reaction may be brought about by treating freshly precipitated calcium 

 carbonate with a stream of chlorine in water : 2'Clj + CaCO 5 = CO 2 + Ca01 2 +,CI 2 O. From 

 this we may conclude that, although carbonic anhydride displaces hypochlorous anhy 

 dride, it may be itself displaced by an excess of the latter. 



