Final answer:
The temperature of water stays constant during boiling because heat is being used to convert water from liquid to gas, a process that requires breaking intermolecular forces and does not increase temperature. The temperature only rises until the boiling point is reached, and further heat input only intensifies boiling without raising temperature.
Step-by-step explanation:
The temperature remains constant during the boiling of water because the heat supplied is used for the phase change from liquid to gas rather than increasing the temperature.
This phase change absorbs energy to break the intermolecular forces that hold water molecules together in the liquid state, which is also known as the heat of vaporization. Hence, during boiling, all the heat being added to the water is consumed in converting the water to steam while keeping the temperature at the boiling point, a phenomenon represented on a heating curve for water.
As such, if more heat is provided by increasing the burner setting, the process of boiling simply becomes more vigorous, but the temperature of the water remains at 100°C under standard atmospheric pressure. Therefore, whether the boiling is gentle or vigorous, the temperature does not increase beyond the boiling point until all the liquid has been converted to gas.