334 SMITH'S INTERMEDIATE CHEMISTRY 



decomposition, such as all acids and salts exhibit when brought 

 in contact with one another: 



CaC0 3 + 2HC1 <= CaCl 2 + H 2 C0 3 . 



The calcium chloride CaCl 2 remains dissolved in the water con- 

 tained in the hydrochloric acid. The carbonic acid H 2 C0 3 is 

 unstable, however, and immediately dissociates into water, which 

 lemains, and carbon dioxide gas, which escapes: 



The apparatus used is similar to that employed in making chlorine 

 (Fig. 49, p. 141). 



2. For commercial purposes the carbon dioxide is either used 

 as it is produced, or else it is compressed into wrought-iron cylin- 

 ders and shipped in the form of a liquid. Three sources of such 

 commercial carbon dioxide are in use : 



When carbon, for example, in the form of coke, is burned with 

 a plentiful supply of air, all the carbon is converted into carbon 



dioxide: 



C + O 2 -C0 2 . 



Since, however, there are four volumes of nitrogen to one of 

 oxygen in the air, this carbon dioxide is diluted with nitrogen 

 in the same proportion. The gases must therefore be separated 

 by leading them through potassium carbonate solution, which 

 absorbs the carbon dioxide : 



C0 2 + H 2 ? H 2 C0 3 + K 2 C0 3 <=* 2KHC0 3 . 



The bicarbonate of potassium KHCO 3 thus produced is subse- 

 quently decomposed by heating. 



3. Calcium carbonate (limestone) CaCO 3 , or, more easily, mag- 

 nesium carbonate (magnesite) MgCO 3 , may be decomposed by 

 heating in a kiln : 



CaC0 3 ->CaO 

 MgC0 3 ->MgO 



