The reaction between hydrogen iodide (HI) and water is acidic because hydrogen iodide is a strong acid. Here's a breakdown:

* HI is a strong acid: This means it readily ionizes (separates into ions) in water, donating a proton (H+) to form hydronium ions (H3O+).

* The reaction:

* HI(aq) + H2O(l) → H3O+(aq) + I-(aq)

* Hydronium ions (H3O+) are responsible for acidity: The presence of these ions in solution leads to a lower pH, indicating acidity.

In summary: The reaction between HI and water results in the formation of hydronium ions, which are responsible for the acidic nature of the solution.