Why Can Water Boil Above 100°C in a Pressure Cooker?

We often say that water boils at 100°C, but that statement assumes a particular condition: the surrounding pressure is close to one standard atmosphere. Boiling begins when a liquid's vapour pressure becomes high enough to match the pressure around it. If the surrounding pressure is higher, the liquid must reach a higher temperature before that condition is met.

A pressure cooker traps steam and raises the pressure inside the vessel. The boiling point of water therefore rises as well. OpenStax explains that at higher pressure, water can remain liquid at temperatures above 100°C, which is one reason pressure cookers can cook food more quickly.

The reverse happens at high altitude, where lower atmospheric pressure allows water to boil below 100°C. So 100°C is not an intrinsic, fixed boiling point of water. It is the boiling point under a particular pressure. The more general relationship is that boiling occurs when the liquid's vapour pressure equals the external pressure.

https://openstax.org/books/university-physics-volume-2/pages/1-5-phase-changes
https://openstax.org/books/chemistry/pages/10-3-phase-transitions