Name the phenomenon as a reason of which water has unusual boiling point.

Answer

**Step-by-Step Solution:**

1. **Understanding Boiling Point:** The boiling point of a substance is the temperature at which it changes from a liquid to a gas. For most substances, this temperature is determined by the strength of the intermolecular forces present.

2. **Identifying Water's Boiling Point:** Water has an unusually high boiling point (100°C at 1 atm pressure) compared to other similar-sized molecules. This indicates that there are strong intermolecular forces at play.

3. **Recognizing Intermolecular Forces:** The key to understanding the high boiling point of water lies in the type of intermolecular forces it exhibits. In the case of water, the primary intermolecular force is hydrogen bonding.

4. **Explaining Hydrogen Bonding:** Hydrogen bonding occurs when a hydrogen atom covalently bonded to a highly electronegative atom (like oxygen) is attracted to another electronegative atom. In water, each hydrogen atom has a partial positive charge (δ+) while the oxygen atom has a partial negative charge (δ-).

5. **Conclusion:** The presence of these strong hydrogen bonds between water molecules requires a significant amount of energy to break, which results in water having a higher boiling point than expected for a molecule of its size. Therefore, the phenomenon responsible for the unusual boiling point of water is **intermolecular hydrogen bonding**.

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