ELEMENTARY CHEMICAL PHYSIOLOGY 



255 



Urea 



Urea is commonly regarded as the diamide of carbonic acid, 

 Carbonic acid. Urea. 



ro 



but there is now a certain amount of evidence to show that in reality 



/NH 3 



it is isocarbamide with a formula HN : C < I 



\0 



In common with other acid amides, urea has weak basic properties, 

 forming unstable salts with nitric and oxalic acids. 



EXPERIMENT I. To some urine, which has been evaporated to 

 small bulk on a water-bath, add some pure, colourless (not fuming) 

 nitric acid, and cool the mixture by holding the test tube under the 



o 



FIG. 183. LJrea nitrate. 



FIG. 184. Urea oxalate. 



tap. Crystals of urea nitrate separate out. Examine these with 

 the microscope, and note that they are either rhombic tables or 

 six-sided plates, which overlap each other like the tiles of a roof 

 (see Fig. 183). 



EXPERIMENT II. Repeat experiment with a saturated alcoholic 

 solution of oxalic acid, and note that the crystals are not unlike 

 those of the nitrate, being elongated plates with bevelled pointed 

 ends (Fig. 184). 



Urea is decomposed by nitrous acid HN0 2 carbonic acid gas 

 and nitrogen being evolved : 



00 <NH: + 8 = N - OH = co * + 2N * + 3H *- 



EXPERIMENT III. Add some fuming nitric acid (i.e. containing 

 nitrous acid) to urine, and note the effervescence which results. 

 That one of the gases evolved is carbon dioxide can be proved by 



