HI(aq) is the symbol for hydroiodic acid, an aqueous solution of hydrogen iodide, HI. Hydrogen iodide is a gas at standard temperature and pressure. Its molar mass is 127.9 grams per mole, and its solubility in water is 245 grams per milliliter.
Hydroiodic acid is classified as a strong acid. It is used in organic chemistry as a reducing agent and as a source of iodine. Hydrogen iodide is industrially obtained from the reaction of molecular iodine with hydrazine, N2H4. The reaction also produces molecular nitrogen as a secondary product. Hydroiodic acid is spontaneously oxidized in the presence of molecular oxygen. It is listed as a controlled substance, since it can be used to produce methamphetamine, an illicit drug.