126 NON-METALS AND THEIR COMBINATIONS. 



Upon cooling the mixture to the freezing-point, acid potassium 

 tartrate separates, while hydriodic acid remains in solution : 

 KI + H 2 C 4 H 4 O 6 : KHC 4 H 4 O 6 + HI. 



Whilst hydriodic acid itself is not of much importance, many of 

 its salts, the iodides, are of great interest. 



Tests for iodine and iodides. 

 (Potassium iodide, KI, may be used.) 



1. Add to free iodine (or to an iodide, after it has been decomposed 

 by a few drops of chlorine water, or by strong nitric acid) mucilage of 

 starch : a dark-blue color is produced, due to the formation of " blue 

 iodized starch." 



2. From solution of an iodide liberate iodine by adding chlorine 

 water, or nitric acid containing a little nitrous acid, and shake the 

 solution with disulphide of carbon or chloroform. After standing a 

 few minutes the liquids form a layer of a beautiful violet color. 



3. Add to solution of an iodide, solution of silver nitrate : a pale 

 yellow precipitate of silver iodide, Agl, falls, which is insoluble in 

 nitric acid, and sparingly soluble in dilute ammonium hydroxide. 



4. Add lead acetate to a neutral solution of an iodide : a yellow 

 precipitate of lead 'iodide, PbI 2 , is produced. 



5. Add mercuric chloride to a neutral solution of an iodide: a red 

 precipitate of mercuric iodide, HgI 2 , is produced. 



6. Add sulphuric acid : violet vapors of iodine are evolved. 



lodic acid, HI0 3 . When iodine is dissolved in strong nitric acid, this solu- 

 tion being then evaporated to dryness and heated to about 200 C. (392 F.) 

 a white residue remains, which is iodine pentoxide : 



61 + 10HN0 3 = 5N 2 O 2 + 5H 2 O + 3I 2 O 5 . 

 By dissolving this oxide in water, iodic acid is obtained : 

 I 2 5 + H 2 = 2HI0 3 . 



Iodic acid is a white crystalline substance, very soluble in water. From 

 iodic acid or from iodates, sulphurous acid and many other reducing agents 

 liberate iodine. 



Sulphur iodide, Sulphuris iodidum, S 2 I 2 . When the two elements, sulphur 

 and iodine, are mixed together in the proportion of their atomic weights, and 

 this mixture is heated, direct combination takes place. The fused mass is 

 grayish-black, brittle, has a crystalline fracture and a metallic lustre. It is 

 almost insoluble in water, but soluble in glycerin and in carbon disulphide. 



Fluorine, F = 19. This element is found in nature, chiefly as 

 fluorspar, calcium flouride, CaF 2 ; traces of fluorine occur in many 



