# Why is the normal boiling point of HF (19.5 ""^@C), so high compared to that of HCl (-85.0 ""^@C)?

For the same reason that water is a liquid at room temperature and pressure, whereas ${H}_{2} S$, ${H}_{2} S e$, and ${H}_{2} T e$ are gases.
So the simple reason underlying these phenomena is hydrogen bonding. Fluorine is a smaller, more polarizing atom than chlorine. Charge separation in the $H - F$ molecule should be greater, and the bond strength of $H - X$ should thus be greater when $X = F$. Of course, this principle manifests in the acid strength of $H C l$ versus $H F$. $H F$ is a poor acid in water.
Because there is greater polarity in the $H - F$ bond, the opportunity for hydrogen bonding in the bulk solvent is greater. The best metric for this INTERMOLECULAR force is the boiling point. $H F$ has a normal boiling point of $19.5$ ""^@C, whereas that of $H C l$ is $- 85.0$ ""^@C. Also compare the boiling points of water versus ${H}_{2} S$, and ${H}_{2} S e$.