CALCIUM AND ITS COMPOUNDS 391 



Washing soda Na2CO 3 ,10H 2 O is added to precipitate both kinds 

 of hardness: 



Ca(HC0 3 ) 2 + Na 2 C0 3 - CaCO 3 1 + 2NaHC0 3 

 CaS0 4 + Na 2 C0 3 -> CaC0 3 1 



The small amounts of salts of sodium which remain in the water 

 have no action on soap. 



Household Ammom^NH 4 OH acts like sodium hydroxide 

 (p. 390): 



Ca(HC0 3 ) 2 + 2NH 4 OH - CaCO 3 1 + (NH 4 ) 2 CO 3 + 2H 2 O 

 CaS0 4 + (NH 4 ) 2 C0 3 -* CaC0 3 i + (NH 4 ) 2 S0 4 



except that it will not precipitate magnesium-ion (see p. 539). 



When borax Na 2 B 4 7 ,10H 2 O (p. 363) is added, it is hydrolyzed 

 and the sodium hydroxide contained in its solution acts as already 

 described. 



The supposed bleaching or whitening action of borax or soda 

 is a myth; these salts prevent staining by the iron in the water. 

 They simply precipitate the iron (present as Fe(HCO 3 ) 2 ), which 

 almost all waters contain, as FeC0 3 before the goods are put in. 

 This precipitate is easily washed out in rinsing. The palmitate, 

 etc., of iron, however, which the soap itself would throw down, is 

 sticky and adheres to the cloth. The air subsequently oxidizes 

 it and gives hydrated ferric oxide (rust), which is brownish-red. 



It is evident that, properly to achieve their purpose, the soda 

 and borax must be added, must be completely dissolved, and 

 must be allowed to produce the precipitation of FeCO 3 , CaCO 3 , 

 etc., all before the soap, or the goods, is introduced. If the soap 

 is dissolved before or with the soda, it will take part in the pre- 

 cipitation, and give sticky particles containing the iron and cal- 

 cium salts of the soap acids. 



Washing powders are, or ought to be, mainly sodium carbonate, 

 mixed with more or less pulverized soap. 



