What does a plateau on a heating curve indicate?

• A. Plateau indicates a phase change where temperature remains constant
• B. Temperature continues to rise during a plateau
• C. The substance changes composition chemically
• D. A plateau indicates the end of heating

Answer: (A)

When heat is added partway up a heating curve, a plateau shows that a phase change is happening and the temperature stays the same. During melting or boiling, energy goes into overcoming intermolecular forces to change the substance’s state rather than increasing the average kinetic energy of its molecules. This energy is latent heat of fusion for melting or latent heat of vaporization for boiling. Once the phase change is complete, heating raises the temperature again. So the plateau indicates a physical phase change, not a chemical reaction or the end of heating.