CALCIUM AND ITS COMPOUNDS 389 



of an inch of crust will increase the bill for fuel by 50 per cent. 

 In addition to this the iron is heated to a higher temperature 

 and may even become red hot. In consequence, it combines 

 more rapidly with oxygen on the outside and displaces hydrogen 

 from the water (p. 51) on the inside, giving in both cases Fe 3 O 4 . 

 Thus the life of the boiler is shortened. If the formation of the 

 crust is not prevented, or if the crust is not removed, the boiler 

 may explode and great damage may be done. 



When hard water is used for washing, in the household or laund- 

 ry, much soap has to be dissolved before the necessary lather 

 can be secured. Soap, which consists of a mixture of the sodium 

 salts of several organic acids, such as palmitic acid H.C0 2 Ci 5 H3i 

 (see p. 438), interacts by double decomposition with the salts of 

 calcium and magnesium giving palmitates etc. of these metals. 

 These salts are insoluble and form a " curd." With sodium 

 palmitate Na(CO 2 Ci 5 H 3 i), for example, the action is 



CaS0 4 + 2Na(C0 2 Ci 5 H 3 i) - Ca(CO 2 Ci 5 H 3 i)2 J, + NajS0 4 . 



Not until all the salts causing the hardness have been decom- 

 posed, does the permanent solution of soap which is required for 

 washing begin to be formed. The waste thus involved is often 

 very great and expensive. 



Treatment of Hard Water. Temporary hardness is com- 

 monly removed, on a large scale, by adding slaked lime (made into 

 milk of lime) in exactly the quantity shown by an analysis of the 

 water to be required, and stirring for a considerable time: 



Ca(HC0 3 ) 2 + Ca(OH) 2 - 2CaC0 3 | + 2H 2 0. (1) 



The bicarbonate is neutralized and all the lime precipitated. The 

 latter is removed by filtration. 



Permanent hardness is not affected by slaked lime, but is re- 

 moved by adding sodium carbonate in the necessary proportion: 



CaS0 4 + Na 2 C0 3 -> CaC0 3 J, + Na 2 S0 4 . (2) 



