What occurs to the temperature of a liquid when heat is added during a phase change?

When heat is added to a liquid during a phase change, such as when it is transitioning from a liquid to a gas (boiling) or from a solid to a liquid (melting), the temperature of the liquid remains constant. This phenomenon occurs because the energy being supplied is used to break the intermolecular bonds that hold the molecules in their current state, rather than increasing the kinetic energy of the molecules, which would raise the temperature.

During the phase change, the added heat, known as latent heat, goes into changing the state of the material instead of raising its temperature. For example, when water boils, the temperature remains at 100 degrees Celsius (at atmospheric pressure) until all the water has turned to steam, despite the continued input of heat. Therefore, it is essential to recognize that the temperature does not increase until the phase change is complete, making the correct answer the one that indicates the temperature remains constant during these changes.