UPSC Combined Geo-Scientist, Mains · 2017 · Mains Paper II
Q1(k). Water versus hydrogen sulfide
Question
Explain why H₂O is liquid whereas H₂S is gas at room temperature.
Answer in brief
Water has an extensive, strong hydrogen-bond network, giving a much higher boiling point than hydrogen sulfide.
Key points
- Oxygen is small and strongly electronegative. Each water molecule can donate two and accept approximately two hydrogen bonds, producing a connected liquid network.
- Sulfur is larger and less electronegative, so pure H₂S does not form a comparable strong hydrogen-bond network; weaker dipolar and dispersion interactions dominate.
- Consequently more thermal energy is required to separate water molecules.
The full model answer
As written for the answer booklet: the diagram carries the working, the lines stay short. Read here only; it is not offered as a download.
CoversWaterhydrogen sulfideIntermolecular forces
All parts of Question 1
- 1(a) Van der Waals equation8 marks
- 1(b) Adiabatic expansion of helium6 marks
- 1(c) Arrhenius acids and bases6 marks
- 1(d) Carnot efficiency6 marks
- 1(e) Dimerization and temperature6 marks
- 1(f) Zero-order integrated law6 marks
- 1(g) Van't Hoff factor6 marks
- 1(h) Molar absorptivity6 marks
- 1(i) Units of magnetic moment6 marks
- 1(j) Surface tension of isomers5 marks
- 1(k) Water versus hydrogen sulfide5 marks
- 1(l) Momentum eigenfunction6 marks
- 1(m) Atomic emission selection rules8 marks