Concept: The physical state of a substance (liquid, gas, solid) at a given temperature and pressure is significantly influenced by the strength of intermolecular forces. Stronger intermolecular forces lead to higher boiling points and melting points, making a substance more likely to be liquid or solid at room temperature.

Why (C) is correct: Water (${\mathrm{H}}_2\mathrm{O}$) molecules are capable of forming extensive hydrogen bonds with each other. Hydrogen bonding is a particularly strong type of intermolecular force that arises when hydrogen is bonded to a highly electronegative atom like oxygen, nitrogen, or fluorine. These strong attractive forces require a significant amount of energy to overcome, resulting in water having a relatively high boiling point ($100^{\circ }\mathrm{C}$) and being a liquid at room temperature.

In contrast, hydrogen sulfide (${\mathrm{H}}_2\mathrm{S}$) does not exhibit hydrogen bonding to a significant extent because sulfur is less electronegative than oxygen. The primary intermolecular forces in ${\mathrm{H}}_2\mathrm{S}$ are weaker dipole-dipole interactions and London dispersion forces. Due to these weaker forces, ${\mathrm{H}}_2\mathrm{S}$ has a much lower boiling point ($-60^{\circ }\mathrm{C}$) and is a gas at room temperature.

Option Analysis:

• A) Water has higher molecular weight: The molecular weight of water ($18\text{ g/mol}$) is lower than that of hydrogen sulfide ($34\text{ g/mol}$). If molecular weight were the sole factor, ${\mathrm{H}}_2\mathrm{S}$ would be expected to have a higher boiling point, which contradicts the observation.
• B) Hydrogen sulphide is week acid: While hydrogen sulfide is a weak acid, its acidic nature does not directly explain its physical state (being a gas) at room temperature. The acidity relates to its ability to donate protons, not its intermolecular forces.
• D) Sulphur has high electronegativity than oxygen: This statement is incorrect. Oxygen is significantly more electronegative than sulfur. The high electronegativity of oxygen is crucial for the formation of strong hydrogen bonds in water.

Correct Answer: (C)