Final answer:
Boiling points of hydrogen halides increase from HCl to HI due to larger halide ions and stronger dispersion forces, while HF has an abnormally high boiling point because of its ability to form hydrogen bonds.
Step-by-step explanation:
The boiling points of the hydrogen halides HCl, HBr, and HI increase progressively from HCl through HBr to HI. This trend is observed because the size of the halide ions increases down the group in the periodic table, resulting in stronger dispersion forces owing to an increase in the number of electrons, which in turn leads to higher boiling points. While hydrogen fluoride (HF) has a much higher boiling point due to its ability to form hydrogen bonds, fundamentally different from the dipole-dipole and dispersion forces that impact the boiling points of HCl, HBr, and HI.
Hydrogen bonds have a pronounced effect on the properties of condensed phases; this is particularly evident in the case of HF which can form intermolecular hydrogen bonds, significantly elevating its boiling point compared to other hydrogen halides that cannot form such strong bonds.
As we descend the group from HCl to HI, molecular sizes increase and so do dispersion forces, which have a more considerable influence on boiling point elevations than the slightly decreasing polarities of the molecules. This explains the continuous increase in the boiling points from HCl to HI.