BR OM1NE IODINE FL UORINE. 243 



Also make the test with solution of mercurous nitrate. A green- 

 ish-yellow precipitate of mercurous iodide, Hgl, is obtained. The 

 corresponding chloride and bromide are white and also insoluble. 



4. To the solution of an iodide add some chlorine water, or a few 

 drops of concentrated nitric acid ; iodine is liberated, which, with 

 strongly diluted starch solution, gives a blue color. Iodine in com- 

 bination has no action on starch. Excess of chlorine oxidizes iodine 

 to colorless iodic acid ; hen.ce, the same precaution must be used as 

 given in test 2 for bromides. 



Traces of iodine may be detected readily by the fine violet color 

 given to chloroform or carbon disulphide when the liquid is shaken 

 with them. 



5. Add a little concentrated sulphuric acid to a few granules of an 

 iodide and warm gently. Colorless hydriodic acid gas is liberated, 

 which causes white fumes with the moisture of the air; also free 

 iodine, which may be recognized by its violet vapor. 



Tests 3 and 4 are usually sufficient to identify iodides or hydriodic 

 acid. 



Iodic 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 -f 10HNO 3 == 5N 2 O 2 + 5H 2 O + 3I 2 O 6 . 

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

 IA + 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. 



Hypoiodous acid and its salts are not known. Periodic acid and its salts 

 can be obtained. These oxygen compounds, in marked contrast to those of 

 chlorine, are stable. Iodine pentoxide is the only oxide of the element 

 known. 



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. 



Compounds of iodine with bromine and chlorine. While the affinity 

 between the halogens is feeble, yet a few compounds formed by their union are 

 known ; all of them are unstable, decomposing readily on heating and some 

 also in contact with water. Of some interest is iodine trichloride, IC1 3 , obtain- 

 able as an orange, crystalline substance by passing dry chlorine gas over iodine, 



