Elements of group- 15, 16, and 17 form compounds with hydrogen with general formula \(MH_3\), \(H_2M\), and \(HX\) (called hydrogen halides). The stability of these hydrides, along with \(M-H\) bond strength and basic nature decreases down the group. Boiling point of these hydrides mainly depends on the strength of their intermolecular forces.
Which of the following is liquid at \(20^oC\)?
A. \(HCl\)
B. \(HI\)
C. \(HF\)
D. \(NH_3\)
E. \(HBr\)
Choose the correct answer from the options given below:
Answer & explanation
Correct answer: option 3
The correct answer is Option (3) → C only
A hydride is a liquid at 20°C (293 K) only if its boiling point is at or above that temperature, which needs strong hydrogen bonding:
(A) HCl — gas (b.p. 189 K); no hydrogen bonding.
(B) HI — gas (b.p. 238 K).
(C) HF — liquid at about 20°C (b.p. 293 K).
(D) \( \ce{NH3} \) — gas (b.p. 238.5 K); its hydrogen bonds are weaker than those of HF.
(E) HBr — gas (b.p. 206 K).
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):
Reactivity towards Hydrogen – “… Hydrogen fluoride is a liquid (b.p. 293 K) due to strong hydrogen bonding. Other hydrogen halides are gases.”