CARBON, 93 



Carbon dioxide, CO 2 = 44. (Formerly named carbonic acid, or 

 anhydrous carbonic acid.) This compound is always formed during 

 the combustion of carbon or of organic matter ; also during the decay 

 (slow combustion), fermentation, and putrefaction (processes of decom- 

 position) of organic matter ; it is constantly produced in the animal 

 system, exhaled from the lungs, and given off through the skin. 

 Many spring waters contain considerable quantities of the gas, one 

 single spring in Nauheim, Germany, liberating as much as 3000 

 pounds of carbon dioxide a day. 



By heating, many carbonates are decomposed into oxides of the 

 metals and carbon dioxide. 



Lime-burning is such a process of decomposition : 



CaCO 3 = CaO -f CO 2 . 

 Calcium Calcium 

 carbonate, oxide. 



Another method for the generation of carbon dioxide is the decom- 

 position of any carbonate by an acid : 



CaCO 3 + 2HC1 = CaCl 2 + H 2 O + CO 2 . 

 Calcium Hydrochloric Calcium 

 carbonate. acid. chloride. 



Experiment 9. Use apparatus represented in Fig. 7, page 79. Place about 

 20 grammes of marble, CaCo 3 , in small pieces (sodium carbonate or any other 

 carbonate may be used) in the flask, cover it with water, and add hydrochloric 

 acid through the funnel-tube. The escaping gas may be collected over water, 

 as in the case of hydrogen, or by downward displacement, i. e., by passing the 

 delivery -tube to the bottom of a tube or other suitable vessel, when the carbon 

 dioxide, on account of its being heavier than atmospheric air, gradually dis- 

 places the latter. This will be shown by examining the contents of the vessel 

 with a burning paper, which is extinguished as soon as most of the air has 

 been expelled. 



Examine the gas for its high specific gravity, by pouring it from one vessel 

 into another ; for its power of extinguishing flames, by mixing it with an equal 

 volume of air, which mixture will be found not to support the combustion of 

 a taper notwithstanding that oxygen is contained in it. Add to one portion of 

 the collected gas some lime-water, shake it, and notice that it becomes turbid. 

 Blow air exhaled from the lungs through a glass tube into lime-water, and 

 notice that it also turns turbid. 



Carbon dioxide is a colorless, odorless gas, having a faintly acid 

 taste. By a pressure of 38 atmospheres, at a temperature of C. 

 (32 F.), carbon dioxide is converted into a colorless liquid, which by 

 intense cold ( 79 C., 110 F.) may be converted into a white, 

 solid, crystal! ine, snow-like substance. The specific gravity of carbon 

 dioxide is 1.524; it is consequently about one-half heavier than 

 atmospheric air. 



