# For an acid-base reaction, what is the reacting species in the following acid: Hydriodic acid? How do you determine this?

$H I$ undergoes almost complete ionization in water. How do you determine this? By experiment.
Except for $H F$, ALL of the hydrogen halides in water are strongly acidic: $H I > H B r > H C l$. This difference in acidity is due to the weaker bond overlap of the higher halides, and also to the entropy effect of the smaller halide ions.