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Hydrogen iodide (HI) is a diatomic molecule. Aqueous solutions of HI are known as hydroiodic acid or hydriodic acid, a strong acid. Hydrogen iodide and hydroiodic acid are, however, different chemical compounds: one is a gas at room temperature, the other is an aqueous solution. They are interconvertible. HI is used in organic and inorganic synthesis as one of the primary sources of iodine and as a reducing agent.