Why does the temperature of boiling water remain the same as long as the heating and boiling continue?

Answer 1

A certain amount of heat must be added to water to change it from liquid water to water vapor (steam). After water has been heated from, say room temperature, to 100, all of that energy is being absorbed by the liquid. At 100, more heat is added and the liquid changes to steam. So the temperature will stay at 100 (that is to say the water will continue boiling) until all of it has been changed to the vapor state. Forming steam removes heat from the liquid water and that keeps the liquid at 100 C. After all of the water has been converted to steam there is no more removing heat by steam formation, then the temperature can rise higher.