THE HALOGEN FAMILY 205 



displacement upon the sulphide ion S = , derived from the solu- 

 tion of H^S in water. Sulphur separates in a fine powder, and 

 a solution of hydrogen iodide (hydriodic acid) is formed in ac- 

 cordance with the equation: 



2H+ + S- + I 2 -> 2H+ + 21- + S j. 



The solution is freed from the deposit of sulphur by filtration, and 

 may be concentrated to 57 per cent of hydriodic acid by distilling 

 off the water. 



Properties. Hydrogen iodide is exceedingly soluble in water 

 and fumes strongly in moist air, giving a fog of HI solution. It 

 is colorless. 



The aqueous solution is an active acid. The iodide-ion I~, 

 which it contains, gives, with any soluble salt of silver, a precipi- 

 tate of insoluble yellow silver iodide Agl : 



AgN0 3 + HI -> Agl | + HN0 3 . 



Chlorine-water or bromine-water, added to a solution of this 

 or any other iodide, displaces the iodine : 



C1 2 + 21- -> 2C1- + I 2 . 



The free iodine, even if present in minute amounts, may be recog- 

 nized by shaking the liquid with a few drops of carbon disulphide. 

 The iodine gives a violet solution in the latter. A still more 

 delicate test is the addition of a drop of very thin starch paste, 

 which gives a deep-blue tint with free iodine. Filter paper dipped 

 in starch paste and dried can also be used, by touching it with a 

 drop of the solution containing the free iodine. 



Uses of Iodine and Its Compounds. The alcoholic solution 

 (tincture of iodine), painted over the skin, reduces swellings and 

 inflammation. lodofonn CHI 3 is a solid used for similar purposes, 

 lodothyrin is an organic compound found in the human thyroid 

 gland, as well as that of other animals. An extract of sheeps' 



