Hydrogen iodide (chemical formula, HI) is a heavy, colorless gas with a strong odor. Chemists prepare it by combining hydrogen and iodine at 200 °C to 300 °C in the presence of a platinum catalyst (a substance that increases the speed of a chemical reaction without being consumed by the reaction), or by adding water to a mixture of iodine and red phosphorus.
Hydrogen iodide dissolves readily in water to form hydriodic acid, also called hydroiodic acid, a strong (extremely active) acid. Chemists use hydriodic acid in experiments as a powerful reducing agent. Such a substance gives some of its electrons to other substances during a chemical reaction. The acid is too costly for large-scale industrial use.
