When a material changes state, such as from liquid to gas (a process known as vaporization), the temperature of the material remains constant during the phase transition. This phenomenon occurs because the energy supplied to the system is used to break the intermolecular forces holding the molecules in the current state rather than increasing the temperature.
For example, when water boils and transitions from liquid to gas at its boiling point (100°C at standard atmospheric pressure), the temperature of the water does not increase until all of the water has transformed into steam. Only after the phase change is complete will any additional heat lead to an increase in the temperature of the gas.
This principle applies universally to phase transitions such as melting (solid to liquid) and freezing (liquid to solid) as well. During these transitions, the system's temperature remains constant until the phase change is complete.