During a phase change at constant pressure, what happens to the temperature and what happens to the heat added?

• A. Temperature increases; heat goes into internal energy
• B. Temperature remains constant; heat goes into latent heat
• C. Temperature decreases; heat leaves
• D. Temperature fluctuates

Answer: (B)

At constant pressure, when a substance changes phase, its temperature stays fixed at the transition temperature. The heat you add doesn’t raise the average kinetic energy of the particles; instead it supplies latent heat—energy used to change the phase. This energy goes into breaking bonds during melting or overcoming cohesive forces during vaporization. For example, ice at 0°C absorbs heat to melt, but its temperature remains 0°C until all the ice has melted; the heat added is the latent heat of fusion. Similarly, liquid water at 100°C gains heat to become steam, and the temperature stays at 100°C until all the liquid has vaporized. Only after the phase change is complete does additional heat raise the temperature again.