Re: What is the lowest temperature at which water can boil?

If you refer to the bottom of this page, there is a phase diagram for water. You will see that water does not exist as a liquid below about zero degrees Celcius, except at pressures above atmospheric. Below that temperature water can still "boil" from the solid phase to the vapor phase at low temperatures, but we call this "sublimation", and it does not have the usual characteristics that we associate with boiling. At pressures above atmospheric pressure, water can still be liquid below zero degrees Celsius, but there will be few molecules at sufficient energy (temperature) to break away from the liquid to go to the vapor phase (evaporation), and certainly no boiling.

So the simple answer to your question is: about zero degrees Celcius.

John Link, MadSci Physicist