The boiling point of a compound is the temperature at which its vapor pressure equals the pressure surrounding the liquid and the liquid changes into a vapor. The boiling point of a compound is influenced by several factors, including intermolecular forces, molecular weight, and polarity.

In the case of HI and HCl, both are polar molecules, but HI has stronger intermolecular forces than HCl due to the larger size of the iodine atom. The larger size of the iodine atom results in a greater polarizability of the molecule, which leads to stronger van der Waals forces between HI molecules. These stronger intermolecular forces require more energy to overcome in order for the liquid to vaporize, resulting in a higher boiling point for HI compared to HCl.

Additionally, HI has a higher molecular weight than HCl (127.9 g/mol vs. 36.46 g/mol), which also contributes to its higher boiling point. The heavier molecules of HI have more inertia and require more energy to overcome the intermolecular forces and escape into the vapor phase.

Therefore, HI has a higher boiling point (424 K) compared to HCl (188 K) due to stronger intermolecular forces and higher molecular weight.