Short Answer

The latent heat of fusion is the amount of heat energy required to change a unit mass of a solid into a liquid at its melting point without changing its temperature. It is an endothermic process because energy is absorbed to overcome intermolecular forces in the solid.

For example, when ice melts into water at 0°C, it absorbs heat energy from the surroundings without any rise in temperature. The heat absorbed during this process is called the latent heat of fusion.

Detailed Explanation

Latent Heat of Fusion

Latent heat of fusion is a specific type of latent heat associated with the solid-to-liquid transition. The term “fusion” means melting. During this phase change, energy is supplied to the substance to break the rigid structure of the solid while keeping the temperature constant. This energy does not increase the kinetic energy of particles; instead, it helps particles overcome intermolecular forces to move more freely in the liquid state.

The latent heat of fusion is a fundamental concept in thermodynamics and explains why substances require energy to melt even at their melting point. Every substance has a specific latent heat of fusion depending on the strength of its intermolecular forces.

1. Characteristics of Latent Heat of Fusion

• Energy Absorption: Heat is absorbed by the solid to melt.
• Temperature Constant: During melting, temperature remains constant until the solid completely becomes liquid.
• Depends on Substance: Substances with strong intermolecular forces, like water, require more energy for fusion.
• Measured per Mass: Typically expressed in Joules per gram (J/g) or Joules per kilogram (J/kg).

For example:

• Water: 334 J/g
• Ice melts at 0°C while absorbing this amount of heat per gram.

2. Process of Fusion

During fusion:

1. Heat energy is supplied to the solid.
2. Particles absorb energy to overcome attractive forces holding them in fixed positions.
3. Solid gradually melts into liquid without any rise in temperature.
4. Once all solid has melted, additional heat increases the temperature of the liquid.

This process is endothermic because energy flows from the surroundings to the substance.

3. Relation to Intermolecular Forces

The latent heat of fusion depends on the strength of intermolecular forces:

• Strong hydrogen bonding or ionic bonding → Higher latent heat.
• Weak van der Waals forces → Lower latent heat.

Example:

• Water has strong hydrogen bonds → high latent heat of fusion.
• Metals like lead have metallic bonds → lower latent heat of fusion compared to water.

4. Applications of Latent Heat of Fusion
5. Daily Life

• Ice in drinks absorbs heat from the drink while melting, keeping the drink cool.
• Melting snow absorbs heat from the surroundings, cooling the ground.

1. Industrial Applications

• Ice production and storage.
• Design of refrigeration systems.
• Metallurgy processes where metals are melted.

1. Climate and Environmental Processes

• Melting of glaciers and ice caps involves large energy absorption due to latent heat of fusion.
• Helps maintain temperature stability in natural water bodies during seasonal changes.

5. Measurement of Latent Heat of Fusion

Latent heat of fusion can be measured experimentally using calorimetry:

Where:

•  = heat absorbed (Joules)
•  = mass of the substance (kg or g)
•  = latent heat of fusion per unit mass

This formula helps calculate the energy required to melt solids in laboratory and industrial setups.

Conclusion

The latent heat of fusion is the energy absorbed by a solid to change into a liquid at its melting point without any temperature rise. It is crucial for explaining melting processes, energy requirements in industrial applications, and natural phenomena like ice melting. Its value depends on the type of substance and the strength of intermolecular forces holding particles in the solid state.