Why does water boil at 100°C?

Answer: Vapor pressure equals air pressure

It's a universal constant — Wrong. 100°C is not universal - it only applies at sea level atmospheric pressure. On Mount Everest, water boils at 70°C because of lower air pressure. In a pressure cooker, it can exceed 100°C.

Vapor pressure equals air pressure ✓ — Correct! Boiling happens when water's vapor pressure equals atmospheric pressure. At 100°C at sea level, molecules have enough energy to form bubbles throughout the liquid, not just at the surface. Lower air pressure means less energy needed, so water boils at lower temperatures at high altitudes!

Maximum temperature for water — Wrong. Water can be heated above 100°C under pressure. In fact, superheated steam in power plants can reach over 500°C. 100°C is just the temperature where vapor pressure matches normal atmospheric pressure at sea level.