Which of the following is the least volatile hydrogen halide?

The correct answer is Option (2) → \(HF\)

Volatility is the ease of passing into vapour; the least volatile compound is the one with the highest boiling point. Fluorine is small and the most electronegative element, so HF molecules are joined by strong hydrogen bonds (H–F···H–F). Hydrogen fluoride is therefore a liquid at room temperature (b.p. 293 K), while the other hydrogen halides are gases.

Boiling points: HF 293 K > HI 238 K > HBr 206 K > HCl 189 K. HF has the highest boiling point and is the least volatile.

Why the other options are incorrect

• Option (1) \(HCl\) — Incorrect. No significant hydrogen bonding and the smallest van der Waals forces of HCl, HBr and HI; it has the lowest boiling point (189 K) and is the most volatile.

• Option (3) \(HBr\) — Incorrect. A gas (b.p. 206 K); held only by van der Waals forces.

• Option (4) \(HI\) — Incorrect. The largest of the four, with the strongest van der Waals forces, but still a gas (b.p. 238 K); without hydrogen bonding it boils well below HF.

Note: This question is from the pre-rationalised NCERT book (2022-23). The topic is not there in the latest NCERT book (2026-27), and it is also out of the CUET 2026 syllabus.

NCERT (Pre-rationalised edition, 2022-23):
Anomalous Behaviour of Fluorine – “… The anomalous behaviour of fluorine is due to its small size, highest electronegativity, low F-F bond dissociation enthalpy, and non availability of d orbitals in valence shell. Most of the reactions of fluorine are exothermic (due to the small and strong bond formed by it with other elements). It forms only one oxoacid while other halogens form a number of oxoacids. Hydrogen fluoride is a liquid (b.p. 293 K) due to strong hydrogen bonding. Other hydrogen halides are gases.”