What is latent heat of fusion?

Short Answer

Latent heat of fusion is the amount of heat required to change 1 kilogram of a solid into a liquid at its melting point without changing its temperature. During melting, heat is absorbed only to break the bonds between solid particles, not to raise temperature.

For example, ice at 0°C absorbs heat to become water at 0°C, but its temperature remains constant until all the ice melts. This absorbed heat is known as the latent heat of fusion.

Detailed Explanation :

Latent Heat of Fusion

Latent heat of fusion is a special type of heat energy involved in the change of state from solid to liquid. When a solid substance is heated, its temperature increases until it reaches the melting point. At this point, even if we continue to supply heat, the temperature does not rise further. Instead, all the heat goes into breaking the strong intermolecular bonds that hold the solid particles together.

This hidden heat, which does not raise the temperature but helps the solid turn into a liquid, is called latent heat of fusion. The word “fusion” means melting.

The SI unit of latent heat of fusion is kilojoules per kilogram (kJ/kg).

Meaning of Latent Heat of Fusion

To understand latent heat of fusion, consider the example of ice:

  • Ice at 0°C is heated.
  • It starts melting but stays at 0°C until the entire ice becomes water.
  • All heat supplied is used to loosen the strong bonds between ice molecules.
  • No temperature rise occurs during this period.

This heat required for melting 1 kg of ice at 0°C into 1 kg of water at 0°C is the latent heat of fusion.

For ice, the latent heat of fusion is 334 kJ/kg, which means:

1 kg of ice needs 334 kJ of heat to melt completely at 0°C.

Why Temperature Does Not Increase During Fusion

When a solid changes to liquid:

  • Heat energy is used to overcome the strong attractive forces between particles.
  • This energy does not increase the kinetic energy or particle speed.
  • Hence, the temperature remains constant during the change of state.

Only after complete melting will the temperature of the liquid begin to rise.

This is why ice remains at 0°C while melting, even if heat is continuously supplied.

Process of Fusion

The process of fusion involves several steps:

  1. Heating the Solid

The solid absorbs heat and its temperature rises until it reaches the melting point.

  1. Reaching Melting Point

The temperature stops increasing once the melting point is reached.

  1. Breaking of Bonds

The heat continues to flow but is used entirely to break the strong intermolecular bonds.

  1. Change of State

The solid gradually converts into liquid, but temperature remains constant.

  1. Completion of Melting

Only after the entire solid melts does the temperature of the liquid start rising.

This sequence explains how latent heat supports the melting process.

Examples of Latent Heat of Fusion

  1. Melting Ice

Ice absorbs 334 kJ/kg of heat to become water.

  1. Melting Metals

Metals like iron and aluminum require large amounts of heat to melt due to strong bonds.

  1. Melting Wax

Wax absorbs heat to become liquid, but its temperature remains constant during melting.

  1. Melting Chocolate

Chocolate melts at a specific temperature and absorbs latent heat to change state.

Applications of Latent Heat of Fusion

Latent heat of fusion has many practical applications:

  1. Ice for Cooling

Ice absorbs large amounts of heat from its surroundings while melting. This keeps drinks or food cold for a long time.

  1. Refrigerators

Refrigerators use the principle of latent heat to remove heat from food items and keep them cool.

  1. Snowfall and Climate

During snowfall, freezing water releases latent heat of fusion, warming the surrounding air and affecting weather patterns.

  1. Freezing of Lakes

When surface water freezes, latent heat is released and slows further freezing, protecting aquatic life.

  1. Industrial Uses

Melting of metals in foundries relies heavily on latent heat of fusion.

Importance in Nature

Latent heat of fusion plays an important role in the environment:

  • Helps regulate temperature in cold climates.
  • Makes the freezing of water slow, preventing sudden temperature drops.
  • Maintains stability in the water cycle.

Without latent heat, temperature changes during melting and freezing would be extreme.

Energy Required for Fusion

The energy needed for fusion depends on:

  • Type of substance (strong bonds need more heat)
  • Amount of substance (more mass needs more heat)
  • Purity of the substance (impurities can lower melting point)

These factors determine the exact amount of heat required for a solid to melt.

Conclusion

Latent heat of fusion is the amount of heat needed to change 1 kg of a solid into a liquid at its melting point without changing its temperature. It helps break intermolecular bonds and enables melting. This concept is important in cooling systems, climate regulation, melting processes, and natural phenomena like snowfall and freezing of lakes. Understanding latent heat of fusion helps us explain why temperature remains constant during melting.