Hydrogen Iodide, HI

a compound of iodine and hydrogen. A colorless gas with a pungent odor, it fumes vigorously in moist air, attracting moisture and forming very fine droplets of hydriodic acid. It has a melting point of 50.9°C and a boiling point of 35.9°C. Hydrogen iodide is an unstable compound, dissociating at elevated temperatures (2HI -H₂ + I₂); it is highly soluble in water. It is obtained by the interaction of iodine vapor with hydrogen over a catalyst at a temperature of approximately 500°C.

The aqueous solution of hydrogen iodide (hydriodic acid) is a strong acid, a colorless liquid with a pungent odor, which gradually turns yellow under exposure to light (owing to the precipitation of iodine as a result of oxidation). Hydriodic acid that is sold on the market contains approximately 45 percent HI and has a density of 1.5 g/cm³; it is used for the preparation of iodides.