Concept:The boiling point is governed by intermolecular forces. Molecules lacking hydrogen bonds generally boil at significantly lower temperatures than molecules of similar size that possess them.
Formula:$$ \text{B.P.} \propto \text{IMF Strength} $$
Solution:- Water (\( \text{H}_2\text{O} \)), Ethanol (\( \text{C}_2\text{H}_5\text{OH} \)), and Acetic acid (\( \text{CH}_3\text{COOH} \)) all contain highly polar \( \text{O-H} \) bonds capable of forming strong hydrogen bonding networks. They are all liquids at room temperature.
- Hydrogen sulphide (\( \text{H}_2\text{S} \)) relies on a much less electronegative sulfur atom. It cannot form hydrogen bonds and only experiences weaker dipole-dipole interactions.
- Because its intermolecular forces are so weak, \( \text{H}_2\text{S} \) requires very little heat to vaporize, existing as a gas at room temperature, thus having the lowest boiling point.
Why other options are incorrect:The other options have extensively hydrogen-bonded liquid structures ensuring higher boiling points.
Quality & Fidelity Assurance:
Every question on BeambePrep is rigorously curated against the official PMDC syllabus with zero filler, zero out-of-syllabus content, and zero typos. When an authentic past paper originally contained a historical mistake or ambiguity from the examining board (such as UHS or NUMS), BeambePrep faithfully reflects the original paper while detailing the nuance and scientific consensus in the autopsy above.