2(J4 SMITH'S INTERMEDIATE CHEMISTRY 



which a trace of it confers a strong blue color (test for free iodine, 

 p. 400). These colors are shown only by free iodine the 

 iodides are colorless. 



Chemical Properties. Iodine unites very slowly and in- 

 completely with hydrogen, giving hydrogen iodide. It combines 

 readily with phosphorus (PI 3 ) and with many of the metals, giving 

 iodides. 



Hydrogen Iodide, Preparation. The gas is prepared by 

 the process used for hydrogen bromide. Red phosphorus and 

 iodine are mixed, and water is allowed to fall drop by drop upon 

 the mass (Fig. 25, p. 52) : 



2P + 3I 2 -> 2PI 3 . 



PI 3 + 3H 2 -> SHI + H 3 P0 3 . 



The gas is very dense (HI = 1 + 127 = 128 g. per 22.4 liters, 

 against 28.95 g. for air) and can be collected by upward displace- 

 ment of air. 



The action of sulphuric acid upon an iodide does not give pure 

 hydrogen iodide> although the action Nal + H 2 SO 4 NaHS0 4 

 + HI .does take place (compare p. 202). Hydrogen iodide, being 

 much less stable than even hydrogen bromide, is a more active 

 reducing agent, and reduces the sulphuric acid to hydrogen sul- 

 phide. The odor of this gas is therefore very conspicuous when 

 an iodide is moistened with sulphuric acid: 



H 2 S0 4 + SHI - H 2 S + 4H 2 + 41,. 



The violet vapor of iodine becomes visible if the test-tube is 

 warmed. A rough test for an iodide is afforded by this action. 



-Another method of making hydrogen iodide is frequently 

 employed when a solution of the gas in water is required, and not 

 the gas itself. Powdered iodine is suspended in water, and hydro- 

 gen sulphide gas is introduced through a tube in a continuous 

 stream. The iodine dissolves slowly in the water and acts by 



