254 HISTORICAL INTRODUCTION TO CHEMISTRY CHAP. 



acid. It is decomposed by metals, liberating half its volume of 

 hydrogen, e.g. : 



2 Hg + 2HI->Hg 2 I 2 +H 2 . 



(Mercurous 

 iodide.) 



With ammonia, iodine interacts to form an explosive black 

 compound, nitrogen iodide, NH 3 ,NI 3 . With potash it gives a 

 mixture si potassium iodide and potassium iodate (compare the 

 preparation of potassium chlorate) : 



3l 2 + 6KOH -> 5KI + KIO 3 + 3H 2 O 



(Potass- (Potass- 



ium ium 

 iodide.) iodate.) 



When this mixture is acidified iodine is reproduced, 



(Hydriodic (lodic 

 acid.) acid.) 



but iodic acid, HIO 3 , can be prepared by the action of sulphuric 

 acid on barium iodate, 



Ba(IO 3 ) 2 +H 2 SO 4 -> BaSO 4 + 2HIO 3 

 or by dissolving iodine in nitric acid : 



3l 2 +ioHNO 3 - 6HIO 3 +ioNO + 2H 2 O ; 



when dried at 200 iodic acid loses water and is converted into 

 iodic anhydride, I 2 O 6 . A stable periodic acid, HIO 4 ,2H 2 O, and 

 a very unstable hypoiodous acid, HIO, are also known. 



C. BROMINE 



Bromine, discovered by Balard in 1826, is a volatile red- 

 brown liquid, intermediate in its properties between chlorine 

 and iodine. 



D. CYANOGEN 



Scheele (1782), by the action of oil of vitriol on Prussian blue, 

 obtained a volatile acid, weaker than carbonic acid, to which the 

 French chemists in 1787 gave the name prussic acid. He 



