22 Biological Chemistry. 



Qualitative Analysis of Organic Compounds. 



Carbon. This is a constituent of all organic com- 

 pounds, and its existence is readily revealed by the fact 

 that the majority of organic substances blacken on heating, 

 or when treated with concentrated sulphuric acid. There 

 are a few organic substances which show neither of the 

 above reactions. In these cases, the substance may be 

 heated in a hard glass tube with about ten times its 



O 



weight of copper oxide ; the carbon will then unite with 

 the oxygen of the copper oxide to form carbon dioxide, 

 the presence of which can be detected by leading the gas 

 formed in the reaction through a tube containing lime- 

 water. 



Hydrogen. The hydrogen in organic substances gives 

 rise to water vapour when the substance is burnt ; if the 

 latter is heated in a tube with or without copper oxide, 

 the steam formed in the reaction will condense in the 

 upper part of the tube. (N.B. The substance must be 

 thoroughly dried (see p. 25) before this test is applied.) 



Oxygen. There is no direct test for oxygen. When 

 the percentages of the other elements present in an organic 

 compound have been determined quantitatively, and the 

 total is less than 100, the difference is due to the amount 

 of oxygen present. 



Nitrogen. This is an important constituent of plant 

 and animal bodies ; in most cases its presence can be de- 

 tected by heating the substance with soda-lime (about ten 

 times its weight), when ammonia is evolved, and can be 

 recognized by the smell. This test is not, however, always 

 decisive. In such a case, a small amount of the substance 

 is gently warmed with a clean piece of sodium or potassium 

 in a test-tube ; the metal first melts and is thus brought 

 into intimate contact with the organic substance. The 



