Why does the temperature during the boiling of a liquid remain constant?

The horizontal part of the curve shows that during the boiling of the liquid, its temperature remains constant, however, during the boiling, the liquid receives a certain amount of heat. The amount of heat that a liquid receives during boiling is spent on the formation of steam. In this case, work is done to increase the volume of bubbles and their rise to the surface of the liquid. As a result, the entire amount of heat that is supplied to the system goes to work.