CH. XII.] UREA. 289 



335. Dilute the urea solution with twice its volume of water. 

 Place a drop on a slide, add a drop of pure nitric acid, cover with a 

 slip, and examine the crystals of urea nitrate that separate out. 

 They form octahedral, lozenge-shaped, or hexagonal plates, often 

 striated and imbricated. Draw the crystals. 



336. Powder two or three crystals of urea in a watch-glass : 

 rub with a small amount of acetone and warm gently on a water 

 bath. The urea dissolves. Allow most of the acetone to evaporate 

 away, and then place a drop of the remaining solution on a watch- 

 glass. Urea crystallises out as the acetone passes off. Draw the 

 crystals. 



337. Repeat the above exercise, using strong alcohol instead 

 of acetone. Draw the crystals of urea, which are usually very 

 irregular. 



338. Dilute the remainder of the aqueous solution left from 

 Ex. 335 with an equal quantity of water, and to a portion of this in 

 a test-tube add some yellow nitric acid (or nitric acid to which a 

 little potassium nitrite has been added). An effervescence and 

 evolution of gas take place. 



CO(NH 2 ) 2 + 2 HN0 2 = C0 2 + 2N 2 + 3 H 2 O. 



NOTE. All compounds containing the amino group (NHj) react in a 

 similar manner when treated with nitrous acid (see Ex. 81). The decompo- 

 sition of urea with nitric acid is relatively very slow. 



339. To another portion of the solution add sodium hypo- 

 bromite. A marked effervescence and evolution of gas take place. 



CO(NH 2 ) 2 + 3NaBrO + 2NaHO 



= 3NaBr + N^CC^ + 3H 2 + N 2 . 



340. To a few cc. of saturated ammonium sulphate add 

 sodium hypobromite. A marked effervescence and evolution of gas 

 take place. 



(NH 4 ) 2 SO 4 + 3NaBrO + 2NaHO 



= Na 2 SO 4 + 5H 2 O + aNaBr + N 2 . 



NOTES. i. All ammonium salts and all compounds with the amino 

 group give off nitrogen when treated with an alkaline solution of sodium hypo- 

 bromite. 



