Q.Why is H2O a liquid and H2S a gas ?

Step 1 — Compare O and S properties.

Oxygen: small atomic radius, very high electronegativity (3.44). Sulphur: larger radius, lower electronegativity (2.58).

Step 2 — Intermolecular forces in $H_2O$.

The high electronegativity and small size of O make the O–H bond strongly polar and allow the lone pairs on O to hydrogen-bond with H atoms of neighbouring molecules:

This extensive 3-D hydrogen-bonded network requires substantial energy to break, so water has an unusually high boiling point.

Step 3 — Absence of hydrogen bonding in $H_2S$.

Sulphur's larger size and lower electronegativity make the S–H bond only weakly polar, insufficient to support hydrogen bonding. $H_2S$ molecules interact only via weak van der Waals forces, requiring little energy to separate.

Step 4 — Result. …

Compare O and S on size and electronegativity, determine whether hydrogen bonding is possible for each hydride, and connect intermolecular force stren …

• Don't attribute the difference to molar mass ($H_2S$ is heavier than $H_2O$ yet boils far lower) — the deciding factor is intermolecular force type, not mass. …