Fig. 60
Carbon forms two oxides, namely, carbon dioxide (CO2) and carbon monoxide (CO).
Carbon dioxide (CO2). Carbon dioxide is present in the air to the extent of about 3 parts in 10,000, and this apparently small amount is of fundamental importance in nature. In some localities it escapes from the earth in great quantities, and many spring waters carry large amounts of it in solution. When these highly charged spring waters reach the surface of the earth, and the pressure on them is removed, the carbon dioxide escapes with effervescence. It is a product of the oxidation of all organic matter, and is therefore formed in fires as well as in the process of decay. It is thrown off from the lungs of all animals in respiration, and is a product of many fermentation processes such as vinegar making and brewing. Combined with metallic oxides it forms vast deposits of carbonates in nature.
Preparation. In the laboratory carbon dioxide is always prepared by the action of an acid upon a carbonate, usually calcium carbonate, the apparatus shown in Fig. 39 serving the purpose very well. This reaction might be expected to produce carbonic acid, thus:
CaCO3 + 2HCl = CaCl2 + H2CO3.
Carbonic acid is very unstable, however, and decomposes into its anhydride, CO2, and water, thus:
H2CO3 = H2O + CO2.
The complete reaction is represented by the equation
CaCO3 + 2HCl = CaCl2 + CO2 + H2O.