180 NON-METALS AND THEIR COMBINATIONS. 



d. Insert a burning stick of wood into a pint bottle or flask, mouth down, 

 until flame is extinguished ; then remove the stick (is it charred?), pour into 

 the bottle about 50 c.c. of lime-water and shake it. Explain results. 



e. Heat slowly about 2 c.c. concentrated sulphuric acid in a dry test-tube with 

 a bit of starch, sugar, Rochelle salt, and wood respectively. Describe and 

 explain the results. 



Carbon dioxide, CO 2 . (Formerly named carbonic acid, or anhy- 

 drous 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 (process 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, a 

 part of which escapes from the water as it rises to the surface. 



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. 



The reason the action takes place readily and proceeds to completion 

 is because carbonic acid is only slightly soluble in water and easily 

 breaks up into water and carbon dioxide gas, which escapes and is 

 removed from the field of action, thus permitting the decomposition 

 to be constantly renewed. Almost any acid will liberate carbon di- 

 oxide from a carbonate. The same principle is involved here as in 

 the case of the liberation of nitrous acid from nitrites, nitric acid 

 from nitrates, sulphurous acid from sulphites, or hydrochloric acid 

 from chlorides. (See Reversible Actions, p. 114.) 



Experiment 12. Use apparatus represented in Fig. 38, page 146. 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 taper, which is extinguished as soon as most of the air has 

 been expelled. 



