What does a plateau on a heating curve indicate?

• A. The curve ends
• B. Temperature rises steadily
• C. Temperature decreases during plateau
• D. Plateaus indicate phase changes where energy goes into changing state

Answer: (D)

Plateaus on a heating curve show that the substance is undergoing a phase change. When heat is added at a melting point, energy goes into breaking apart the solid’s structure so it becomes a liquid, not into raising temperature. That energy is the latent heat of fusion, and the temperature stays the same until all the solid has melted. The same idea applies at the boiling point: heat goes into turning liquid into gas, so the temperature remains constant during vaporization—the latent heat of vaporization. Only after the phase change is complete does the temperature start to rise again. This is why a plateau indicates a state change where energy goes into changing the state rather than increasing kinetic energy.