Which of the following bond types is responsible for the high boiling point of water?

1. Analyze the question The question asks which bond type explains why water has an unusually high boiling point (100°C) compared to similar molecules such as H₂S (about -60°C), H₂Se, and H₂Te.

2. Explain the concept

• Covalent bonds are the bonds within a water molecule (O-H). They hold the atoms together but are not broken when water boils. Boiling only separates the molecules from one another.
• Hydrogen bonds are the attractions between water molecules. They form because oxygen is very electronegative, so the O-H bond is highly polar. The hydrogen carries a partial positive charge (δ+) and is attracted to a lone pair on the oxygen (δ-) of a neighbouring molecule.
• Hydrogen bonding is the strongest type of intermolecular force. A lot of extra heat energy is needed to overcome these attractions, so the boiling point is high.

3. Solution, step by step

1. Boiling means molecules escape from the liquid into the gas phase.
2. This requires overcoming the forces between molecules (intermolecular forces), not breaking the bonds within them.
3. In water, the main intermolecular force is hydrogen bonding.
4. Check the other options: • A. Metallic bond: found in metals, not in water. • B. Covalent bond: it is present in water, but it is intramolecular and does not explain the boiling point. • C. Ionic bond: water is a covalent compound, so this is not relevant. • D. Hydrogen bond: the attractions between water molecules, which explain the high boiling point.

4. Answer The correct option is **D.

5. Exam tip Remember "FON": hydrogen bonding occurs when H is bonded to F, O, or N. If a question asks about unusually high boiling points of H₂O, HF, or NH₃, think hydrogen bonding. Also remember that covalent bonds are within molecules and hydrogen bonds are between molecules.