During a phase change at constant temperature, what happens to the temperature?

• A. Temperature continues to rise.
• B. Temperature continues to fall.
• C. Temperature remains constant.
• D. Temperature fluctuates.

Answer: (C)

When a substance changes phase at a fixed temperature, the temperature stays the same. The added energy goes into changing the state (latent heat) rather than increasing the average kinetic energy of the particles.

During melting, energy is used to break the solid’s orderly structure so it becomes a liquid; during boiling, energy is used to overcome intermolecular forces so it becomes a gas. At the melting or boiling point, both phases can exist in equilibrium, so the temperature remains constant until the phase change is complete. After that, heating or cooling resumes changing the temperature. For example, ice at 0°C stays at 0°C while it melts, and water at 100°C stays at 100°C while it boils.