Why doesn't boiling water keep getting hotter?
During steady boiling at a fixed pressure, added heat mainly turns liquid water into vapor. The boiling temperature itself depends on pressure, so it is not always 100 °C.
- The scientific idea
- Boiling
- Fields of science
- Physics
How it works
For pure water at one standard atmosphere, the familiar boiling temperature is about 100 °C. Lower the external pressure, as happens at higher altitude, and water boils at a lower temperature. The number belongs to a set of conditions, rather than to water in every situation.
Look closer
Once an open pot of water is boiling steadily at a fixed pressure, extra heating mainly makes more vapor instead of continually raising the liquid's temperature. Energy is going into the change of state. Evaporation can happen quietly at the surface well below this point; boiling gives vapor a route out from within the liquid. Those two ways of becoming gas are related, but they are not interchangeable terms.