Question

$HCl$ is gas but $HF$ is a low boiling liquid. This is because
A. H−F bond is strong
B. H−F bond is weak
C. Molecules aggregate because of hydrogen bonding
D. $HF$ is a weak acid

Answer 1

Hint: Halogen hydrides are covalent compounds. Therefore both hydrogen chloride and hydrogen fluoride are covalent compounds. But owing to the small size and very high electronegativity of fluorine it can form other bonds as well.

Complete Step by Step Answer:
Hydrogen chloride is formed due to the reaction of hydrogen atom and chlorine atom. Hydrogen atoms contain only one electron in its valence shell and chlorine atom also contains one electron less from noble gas configuration in its valence shell, so hydrogen and chlorine atoms share their electrons and form a covalent bond between them. Thus it exists as a gas.

Hydrogen fluoride has a covalent bond between hydrogen atom and fluorine atom. Due to the high electronegativity difference between $F$ and $H$, $HF$ can form hydrogen bonds and this bond is the strongest bond among all hydrogen halides. Thus due to formation of hydrogen bonding hydrogen fluoride exists as a liquid.
Thus the correct option is C.

Note: The reactivity of the halogens increases from iodine to fluorine. Thus chlorine is less reactive than fluorine. Due to high reactivity hydrogen fluoride is more acidic than hydrogen chloride.