Question

why temperature remain constant during boiling

Answer 1

An interesting property of matter is that its temperature will remain constant during a phase change, assuming its surrounding pressure is constant.

In other words, once the temperature of a liquid reaches its boiling point, the temperature will not increase until all the liquid has changed its phase to become a gas. Likewise, once the temperature of a liquid is lowered to its freezing point, the temperature will not decrease until all the liquid has changed its phase to become a solid. A similar effect occurs in the opposite direction.

Temperature is the average kinetic energy (KE) of the material's molecules. That means for a given temperature there will be some molecules moving faster and having a higher kinetic energy, while others move at a slower speed.

Temperature does not necessarily remain constant in transitions to exotic states.

Answer 2

this is because the external temperature remain constant. if you increase pressure then the temperature goes up and if you decrease pressure then temperature decreases.