Question

Why $~HI$ has a higher boiling point than $HCl.$

Answer 1

We know that the group of the Hydrogen halides are the specific diatomic inorganic compounds with the common chemical formula $HX$ where $X$ is the one of the halogens among the fluorine, chlorine, bromine, iodine, or astatine.

Complete answer:
As we know, the Hydrogen Halides. Then we have to give a proper reason to showcase the reason that could be a deciding factor in the abnormality of boiling points trend which in case could be hydrogen bonding or intermolecular forces. The basic properties of the Hydrogen halides are that they are the gases that indeed dissolve in the water in order to give the acids [citation needed] as the products which are commonly known as hydrohalic acids. The boiling point of a given molecule depends upon the intermolecular interactions. Higher the strength of these interactions, higher is the value of boiling point of molecules.
The direct reaction of hydrogen with fluorine and chlorine gives hydrogen fluoride and hydrogen chloride, respectively. Industrially these gases are, however, produced by treatment of halide salts with sulfuric acid. The boiling point rises from $~HCl$ to $HI.$ The increasing intensity of the van der Waals intermolecular forces is due to this trend, which coincides with the number of electrons in the molecules. The HI molecule’s diffuse electron cloud makes for intermolecular interaction that is more successful. $HI$ should have a higher boiling point/melting point than $~HCl$ provided this improved intermolecular interaction.

Note:
Remember that the boiling points of $HF$ and $HI,$ can occur because $HF$has a higher boiling point than the HI. HI has the larger size, and greater van der Waals forces but there is the presence of hydrogen intermolecular interactions in $HF.$