What is latent heat of fusion and latent heat of vaporization?

• A. Latent heat of fusion is energy to melt 1 mole (or kg) of solid at its melting point; latent heat of vaporization is energy to vaporize 1 mole (or kg) of liquid at its boiling point.
• B. Latent heat of fusion is energy to melt a liquid.
• C. Latent heat of vaporization is energy to heat a solid.
• D. Latent heat of fusion is energy to vaporize a solid.

Answer: (A)

Latent heat is the energy absorbed or released when a substance changes phase at a constant temperature. For melting a solid at its melting point, the needed energy goes into breaking and rearranging the solid’s organized structure so it becomes a liquid—that amount is called the latent heat of fusion, and it is typically specified per mole or per kilogram. For boiling a liquid at its boiling point, the required energy provides enough energy to completely overcome intermolecular forces to form a gas, which is the latent heat of vaporization (also given per mole or per kilogram). These two quantities reflect two distinct phase-change processes: solid to liquid at the melting point, and liquid to gas at the boiling point, with no temperature change during the transition.

Other descriptions aren’t accurate descriptions of the standard latent heats. Heating a solid describes sensible heat, where the temperature changes. “Melting a liquid” isn’t a standard phase-change term, and vaporizing a solid would be sublimation, a different latent heat.