Does the temperature always change during a phase change?

• A. No. During a phase change, temperature stays constant while energy is used to change the state.
• B. Yes; temperature changes during a phase change.
• C. Sometimes; temperature changes during a phase change for some substances.
• D. It depends on the amount of energy added.

Answer: (A)

During a phase change, energy goes into changing how the particles are arranged rather than increasing their average speed. This energy is latent heat of fusion or vaporization, used to break or form intermolecular forces as the substance melts or boils. Because that energy is spent on changing the state, the temperature stays the same until the phase change is complete. Only after all of the material has transformed does additional energy start to raise temperature again. A heating curve shows flat regions at the melting and boiling points, illustrating this constant temperature during the phase transition. The amount of energy added affects how much of the substance has changed state and how long the plateau lasts, not the temperature itself during the transition.