THE CARBONATES Hi 



which is decomposed by heat, and acted upon by lime, as 



represented in the equations : — 



CaCOg + H2CO3 = CaH22C03 

 CaH22C03 = CaCOg + HgO -f COg 

 CaH22C03 + Ca20H = 2CaC03 + 2H2O. 



Hardness of Water. — Although the acid carbonate of calcium has, 

 owing to its instability, never been isolated, the acid carbonates or 

 **bicarbonates" of sodium and potassium are well-known and stable salts. 

 (These are produced by passing carbonic acid gas into cold saturated 

 solutions of the carbonates ; they are decomposed into the carbonate and 

 carbonic acid when warmed.) There is therefore good reason to believe that 

 the calcium compound can also exist in solution. Its probable existence 

 affords an explanation of the hardness of spring water which prevents the 

 lathering of soap until all the lime has been deposited as a curd. Such 

 water deposits its chalk on the sides of the kettles or boilers in which it is 

 heated in the form of " fur" ; hence, when the hardness is due to dissolved 

 chalk, it is said to be "temporary." To soften such water on a large 

 scale, lime may be placed in the reservoirs used for storing it. The hard- 

 ness of spring water may also be due to dissolved sulphates or chlorides 

 of calcium and magnesium, especially the sulphate of calcium. These salts 

 are, of course, not precipitated by boiling, and the hardness they produce is 

 said to be "permanent." To soften such water on a small scale, carbonate 

 of sodium may be added, which, in absence of dissolved carbonic acid, 

 completely precipitates the lime or magnesia as carbonate, thus : — 



NagCOg + CaS04 = Na2S04 + CaCOg. 

 As already pointed out on p. 30, spring water obtains its dissolved 

 salts from the soil or rock strata through which it passes. Owing to the 

 carbonic acid in rain water, much of the chalk contained in spring or river 

 water is dissolved from the soil, a fact that explains the necessity of the 

 application of lime or chalk every few years on most land. 



The only other carbonate that need be examined is the 

 carbonate of ammonium. Procure a specimen of " lump 

 ammonia." Notice that it smells strongly of ammonia, 

 showing that even at the ordinary temperature it dissociates 

 into ammonia and carbonic acid. Dissolve a little in water, 

 and to the solution add a solution of gypsum. Note that 

 chalk is precipitated : — • 



{NH4)2C03 + CaS04 =-- (^YL^^^O^^C^CO^ 



