At ordinary temperature H$_2$O is a liquid while H$_2$S is a gas. This is because H$_2$O has

Analyze the question: Water (H₂O) and hydrogen sulfide (H₂S) have similar structures, since oxygen and sulfur are in the same group. Yet H₂O is a liquid at room temperature and H₂S is a gas. We need the reason for this difference.

Concept: Boiling point depends on the strength of the forces between molecules.

• Hydrogen bonding is a strong type of dipole-dipole attraction. It occurs when hydrogen is bonded to a small, highly electronegative atom (N, O or F).
• Oxygen is small and highly electronegative (3.5), so the O-H bond is very polar. The partially positive H of one molecule is strongly attracted to the lone pair on the O of a neighbouring molecule.
• Sulfur is larger and less electronegative (2.5), so H₂S cannot form effective hydrogen bonds. Only weak dipole-dipole and van der Waals forces act between its molecules.

Solution:

1. H₂O molecules are held together by hydrogen bonds, which take a lot of energy to break. Its boiling point is 100 °C, so it is a liquid at ordinary temperature.
2. H₂S has only weak intermolecular forces. Its boiling point is about -60 °C, so it is a gas at ordinary temperature.

Checking the options:

• A. Wrong. Weak forces would make water a gas, like H₂S.
• B. Correct. Strong hydrogen bonds hold the water molecules together.
• C. Wrong. Induced dipole-induced dipole (London) forces are weak, and they are not what makes water a liquid.
• D. Wrong. Water is a covalent molecule, and there are no ionic forces between its molecules.

Answer: B

Exam tip: Remember "FON" (Fluorine, Oxygen, Nitrogen). If H is bonded to F, O or N and the question asks about an unusually high boiling point or about a liquid state, the answer is almost always hydrogen bonding. This also explains why ice floats and why water has a high boiling point compared with other Group 6 hydrides.