During a phase change, how does the temperature change?

• A. Temperature increases
• B. Temperature remains roughly constant
• C. Temperature decreases
• D. Temperature fluctuates

Answer: (B)

During a phase change, the temperature stays constant because the added energy goes into changing the state rather than increasing the average kinetic energy of the molecules. When a solid melts or a liquid boils, the energy is used to break intermolecular forces (latent heat of fusion for melting, latent heat of vaporization for boiling). As a result, the temperature remains fixed at the melting point or boiling point until the entire substance has changed phase. Only after the phase change is complete will heating or cooling change the temperature again. That flat plateau on a heating curve shows why the temperature doesn’t rise, drop, or fluctuate during the transition.