Which term describes the energy needed to melt or freeze a substance?

• A. Cohesion
• B. Adhesion
• C. Solar wind
• D. Latent heat of fusion

Answer: (D)

The energy involved in changing a substance between solid and liquid at its melting point is called latent heat of fusion. This energy is needed to break the organized structure of a solid so its molecules can move more freely as a liquid. It’s called latent because it doesn’t raise the temperature during the phase change; the energy goes into changing the phase itself. The same amount of energy is released when a liquid freezes back into a solid. So, melting and freezing rely on latent heat of fusion.

Cohesion and adhesion describe how molecules attract each other within or between substances, not the energy required for a phase change. Solar wind is unrelated to melting or freezing.