472 



PRINCIPLES OF CHEMISTRY 



these substances. It may be prepared in the laboratory by passing a 

 stream of chlorine through a cold mixture of water and lime (milk of 

 lime). The mixture must be kept cold, as otherwise 3Ca(ClO) 2 passes 

 into 2CaCl 2 -fCa(ClO 3 ) 2 . In the manufacture of bleaching powder in 

 large quantities at chemical works, the purest possible slaked lime is. 



FIG. 68.- -Apparatus for the manufacture of l>le:irlihi'j- powder (at -mall work-o by the' action of 

 chlorine, which is generated in the vessels 0, on lime, which is churned him 31. 



taken and laid in a thin layer in large flat chambers, M (whose walls. 

 are made of Yorkshire flags or tarred wood, on which chlorine has no 

 action), and into which chlorine gas is introduced by lead tubes. The 

 distribution of the plant is shown in the annexed drawing (fig. 68). 



acting on the dry mass obtained as above, disengages all the chlorine from it, forming 

 only calcium carbonate. But if the bleaching-powder be obtained by a wet method, or if 

 it be dissolved in water (it is very soluble), and carbonic anhydride be passed into it, 

 then chlorine is no longer disengaged, but chlorine oxide, C1 2 O, and only half of the 

 chlorine is converted into this oxide while the other half remains in the liquid as- 

 calcium chloride. From this it may be supposed that calcium chloride is formed by the 

 action of water on bleaching powder, and this is proved to be the case by the fact that 

 small quantities of water extract much calcium chloride from bleaching-powder. If a 

 large quantity of water act on bleaching-powder there remains an excess of calcium 

 hydroxide, a portion of which is not subjected to change. The action of the water may 

 be expressed by the following formulae : From the dry mass Ca^(HO) 6 Cl, there is formed 

 lime, Ca(HO) 2 , calcium chloride, CaCl 2 , and a saline substance, Ca(ClO) 2 . Ca 5 H 6 O 6 Cl 4 

 = CaH 2 O 2 + CaCloOo + CaCl 2 + 2H 2 O. The resulting substances are not equally soluble ;: 

 water first extracts the calcium chloride, which is the most soluble, then the compound 

 Ca(ClO) 2 , and calcium hydroxide is ultimately left. A mixture of calcium chloride and 

 hypochlorite passes into solution. On evaporation there remains Ca 2 O 2 Cl4,8H 2 O. The 

 dry bleaching-powder does not absorb more chlorine, but the solution is able to absorb 

 it in considerable quantity. If the liquid be boiled, a considerable amount of chlorine 

 monoxide is evolved. After this calcium chloride alone remains in solution, and the 

 decomposition may be expressed as follows: CaCl 2 + CaCl 2 O 2 + 2Cl 2 = 2CaCL + 2CLO. 

 Chlorine monoxide may be prepared in this manner. 



It is sometimes said that bleaching-powder contains a substance, Ca(OH)->CL, like 

 calcium peroxide, CaO 2 , in which one atom of oxygen is replaced by (OH) ... and the other 

 by Clo ; but, judging from what has been said above, this can only be admitted in the- 

 dry mass, and not in solutions. 



After being kept for some time, bleaching-powder sometimes decomposes, with the- 

 evolution of oxygen (page 161) ; the same takes place when it is heated. 



