What happens to the temperature of a substance during a phase change?

During a phase change, the temperature of a substance remains constant even though heat is being added or removed. This is because the energy supplied or removed during a phase change is used to break or form intermolecular bonds rather than to increase the kinetic energy of the particles, which is what temperature measures.

For example, when ice melts to become water, the temperature of the substance remains at 0°C until all the ice has turned to liquid. Similarly, when water boils and turns into steam, the temperature remains at 100°C until all the water has evaporated. This plateau in temperature during phase changes is a fundamental characteristic of thermal processes in substances.