The acid HI, known as hydroiodic acid, is one of a handful of strong acids. A strong acid is defined based on its acid dissociation constant, which measures how much the hydrogen atom dissociates when the acid is added to a solution.
Other strong acids include sulfuric acid, hydrochloric acid, hydrobromic acid, nitric acid and perchloric acid. A strong acid is defined as an acid that has a pKa below -1.74. The pKa is derived from Ka, the acid dissociation constant. In an aqueous solution, HI completely dissociates to join with the H2O to form H3O. Weaker acids do not dissociate fully. Hydroiodic acid can be formed from hydrogen iodide.