Q.Explain why HCl is a gas and HF is a liquid at room temperature.

Strong H-bonding raises HF's boiling point (liquid); HCl has only weak forces (gas).

The physical state at room temperature is decided by the strength of the intermolecular forces.

• In HF, fluorine is the smallest and most electronegative halogen, so the H-F bond is highly polar and the H atom can form strong hydrogen bonds with the fluorine of neighbouring molecules (H---F). These strong intermolecular hydrogen bonds hold the molecules together, so HF has an anomalously high boiling point (about 19.5 C) and is a liquid at room temperature. …