When a liquid starts to boil during heating, its temperature
As a liquid is heated, its temperature increases. As its temperature increases, the molecules of the liquid gain energy and their kinetic energy increases. As the kinetic energy increases, the molecular motion increases and the molecules of the liquid overcome the force of attraction between them.
On continuous heating, a particular temperature is reached where the molecules of the liquid leave the surface in the form of vapour. This produces a pressure above the liquid which is equal to the atmospheric pressure and the liquid starts boiling. At this stage, the temperature of the liquid remains stationary even on further heating. This stationary temperature at which the vapour pressure of the liquid is equal to the atmospheric pressure is called the boiling point of that liquid. At this temperature, bubbles begin to form and rise in the liquid.
Hence when a liquid starts to boil its temperature remains constant.