TIIK HALOGENS 47$ 



The products of the metalepsis of alkaline hydrates, XaCIO and 

 Ca(CK)).,, which are held in solutions of Mavelle salt,' and bleaching 

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

 counted as salts, because their metals are liable to 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 HXO ;> ) into water and the anhydride, or chlorine monoxide, 

 ( < LO = 2HC1O H. 2 O ; 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 the product 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 which is most simple under the action of 

 hydrochloric acid itself, because (p. 456) NaCl + NaCIO + 3HC1 

 =2NaCl + HCl + CL 2 + H,O; (2) oxygen, as we saw in Chapter III. 

 (p. 161) 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 + NaClO==2NaCl + O ; (3) and, lastly, chlorine monoxide, which 

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

 similar acid (in order that hydrochloric acid should not yet be produced) 

 be added to a solution of a bleaching salt (which has an alkaline reac- 

 tion, owing either to an excess of alkali or to the feeble acid properties 

 of HC1O), 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 (p. 371). An excellent method for 

 the preparation of chlorine monoxide is based on these feeble acid pro- 

 perties of hypochlorous acid. Zinc oxide and mercury oxide, under 



31 For this reason it is necessary that in the formation 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 chlorine 

 iiK.nu7.ide, because calcium carbonate also gives chlorine monoxide under the action of 

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

 carbonate with a stream of chlorine in water: aClo + CaCOj^CO.^ + CaCLj + CLO. From 

 this \\e may conclude that, although carbonic anhydride displaces hypochlorous anhy- 

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



