CARBON, SILICON, BORON. 179 



Carbon compounds. The chemistry of the carbon compounds is 

 very extensive and intricate. There are more compounds containing 

 carbon than the total of all other compounds, and for good reasons 

 they are ptudied in a subdivision of chemistry, called Organic Chem- 

 istry. A few simple compounds are taken up in Inorganic Chem- 

 istry because of their similarity in properties to the other inorganic 

 compounds. These are the two oxides of carbon, the carbonates, 

 carbon disulphide, and sometimes the cyanides and sulphocyanates. 



Nearly all animal and vegetable substances contain carbon, and 

 when heated they usually undergo decomposition and char, that is, 

 leave a residue of black carbon. This is because the other elements 

 go off first in various combinations, while carbon remains last. But 

 some of the carbon also passes off as volatile products. If the heat 

 is high enough and air has access, the carbon finally disappears or, as 

 is said, the substance burns up. Carbon compounds that are volatile 

 when heated do not char. For example, alcohol, ether, chloroform, 

 and many others simply vaporize when heated, although they contain 

 carbon. Carbon may be shown to be present in such compounds by 

 burning them in the air (when combustible) under a funnel and draw- 

 ing the products through lime-water, or by causing the vapors to come 

 in contact with copper oxide heated to redness in a tube and passing 

 the products through lime-water. When carbon compounds are 

 burned up, either in air or by oxidizing agents, as copper oxide, the 

 carbon passes off as gaseous carbon dioxide, CO 2 , which unites with 

 lime-water to form insoluble white calcium carbonate. In fact, this 

 is the only common gas which acts in this way with lime-water. 



When carbon compounds containing non-volatile metals are 

 charred, the metals remain as carbonates or oxides mixed with the 

 residue of carbon. Many carbon compounds char when heated with 

 concentrated sulphuric acid because the acid extracts hydrogen and 

 oxygen in the proportions to form water, for which it has a powerful 

 affinity, thus leaving a residue of carbon. 



Experiment 11. a. Heat a little starch on charcoal with the blow-pipe 

 flame. Note that it blackens and burns up completely. 



6. Heat a small knifepointful of sugar in a dry test-tube, gradually increas- 

 ing the temperature. Note the melting, browning condensation of water on 

 the walls of the tube, odor, and final charring. 



c. Heat gradually a little Eochelle salt (K.Na.C 4 H 4 O 6 ) in a porcelain cru- 

 cible held in a triangle. The salt melts, effervesces, evolves inflammable 

 vapors, and chars. Heat finally to redness, cool, and add some dilute acid. 

 Any effervescence ? Explain. Make the same experiment with tartaric acid. 

 Note any difference in results. Note also the odor while heating. 



