Answer 1

1) Due to greater electronegativity of O than S, H₂O undergoes extensive intermolecular H-bonding. As a result, H₂O exists as an associated molecule in which each O is tetrahedrally surrounded by four water molecules. Quite a larger amount of energy is required to break these H-bonds. Therefore, H₂O is a liquid at room temperature.

2) In contrast, H₂S does not undergo H-bonding. It exists as discrete molecules which are held together by weak van der Waals forces of attraction. To break these forces of attraction, only a small amount of energy is required. Therefore, H₂S is a gas at room temperature.