What is Latent Heat of Fusion and Latent Heat of Vaporization?

When a substance changes its state (like ice melting or water boiling), the temperature remains constant even though heat is being added. This "hidden" heat is known as Latent Heat.

The term "latent" comes from the Latin word latere, meaning "to lie hidden," because this energy does not increase the kinetic energy of the particles (which would raise the temperature), but instead works to overcome the forces of attraction between them.

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1. Latent Heat of Fusion

This refers to the heat energy required to change a substance from a solid to a liquid state at its melting point without any change in temperature.

• Process: Melting (Solid $\rightarrow$ Liquid).

• Definition: The amount of heat energy required to change 1 kg of a solid into a liquid at atmospheric pressure at its melting point.

• Example: When ice at 0°C melts into water, it absorbs heat, but the temperature of the ice-water mixture remains at 0°C until all the ice has melted.

• Key Concept: Particles in water at 0°C have more energy than particles in ice at the same temperature because of this absorbed latent heat.

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2. Latent Heat of Vaporization

This refers to the heat energy required to change a substance from a liquid to a gas state at its boiling point without any change in temperature.

• Process: Boiling (Liquid $\rightarrow$ Gas).

• Definition: The amount of heat energy required to change 1 kg of a liquid into a gas at atmospheric pressure at its boiling point.

• Example: When water reaches 100°C, it begins to turn into steam. Even though you continue heating the stove, the thermometer will stay at 100°C until all the water has evaporated.

• Practical Fact: Steam at 100°C causes more severe burns than boiling water at the same temperature. This is because steam has absorbed extra energy in the form of latent heat of vaporization.

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Summary Table

Feature	Latent Heat of Fusion	Latent Heat of Vaporization
Phase Change	Solid to Liquid	Liquid to Gas
Point of Occurrence	Melting Point	Boiling Point
Energy Use	Breaks bonds in solid lattice	Breaks atmospheric pressure and liquid bonds
Temperature	Constant	Constant