During a phase change at constant temperature, where does the added energy go?

• A. It raises the temperature.
• B. It changes the chemical composition.
• C. It goes into breaking or forming intermolecular bonds.
• D. It cannot be stored.

Answer: (C)

When a substance undergoes a phase change at a fixed temperature, the added energy goes into changing the interactions between particles, not into heating them. This energy is used to break intermolecular bonds during melting or vaporization, or to form bonds during freezing or condensation. In other words, it increases the system’s potential energy as the particles rearrange into a different phase, while the average kinetic energy (and thus the temperature) stays the same until the phase change is complete. The chemical identity doesn’t change, and the energy isn’t just “lost” or unused.