Question

Which one of the halogen acids is a liquid?
A. $HBr$
B. $HI$
C. $\,HCl$
D. $HF$

Answer 1

Hint: The hydrogen atom combining with different halogen or group $17$ elements forms halogen acids. The halogen acid which is most stable can exist as liquid.

Complete Step by Step Answer:
Among the halogen acids hydrogen iodide has the lowest bond dissociation energy so, it can easily donate its hydrogen atom and acts as the strongest acid among the halogen acids. Hydrogen fluoride is least acidic among halogen acids.

Among halogen acids as the size of the halogen atoms increases from fluorine to iodine the bond with the small hydrogen atom becomes weak due weak orbital overlap. So, among the halogen acids hydrogen iodide has the weakest bond that is formed between the small hydrogen and big iodine atom. It has the lowest bond dissociation energy among the halogen acids. Thus it has the highest acidity.

In case of hydrogen fluoride it forms intermolecular hydrogen bonding between electropositive hydrogen atom and electronegative fluorine atom. Thus due to the presence of hydrogen bonding hydrogen fluoride exists as liquid.
Thus the correct option is D.

Note: Hydrogen bonds can be defined as the attractive force which binds the hydrogen atom of one molecule with the electronegative atom of another molecule. Hydrogen fluoride forms a strong hydrogen bond and acts as a weakest acid.