What is latent heat of vaporization?

Short Answer:

The latent heat of vaporization is the amount of heat energy required to change a liquid into vapor at its boiling point, without changing its temperature. During this process, the heat is used to break the bonds between liquid molecules, turning them into gas, even though the temperature remains constant.

This concept is very important in thermodynamics, especially in steam generation, boiling, and condensation processes. It explains how substances absorb large amounts of energy during phase change, which is later released when the vapor condenses back into a liquid.

Detailed Explanation:

Latent heat of vaporization

In thermodynamics, when a substance changes its state from liquid to gas, it needs to absorb a certain amount of heat energy to complete this phase change. This energy is called the latent heat of vaporization. The term “latent” means hidden, because this energy does not increase the temperature—it is only used to change the phase.

For example, when water at 100°C is boiled, it requires additional heat to become steam. Even though you keep adding heat, the temperature remains constant until all the water turns into steam. That heat energy being added is what we call the latent heat of vaporization.

What Happens During Vaporization

  • Liquid molecules are held together by attractive forces.
  • As heat is applied, these molecules gain energy.
  • When the boiling point is reached, molecules need extra energy to break free from liquid and become gas.
  • This energy is absorbed without raising temperature, and is stored as internal energy in the vapor phase.

Key Characteristics

  1. No temperature change
    • Even with continuous heating, temperature remains constant during vaporization.
  2. Heat is used to break molecular bonds
    • Not for increasing kinetic energy, but for phase separation.
  3. Units of latent heat
    • Usually expressed in kJ/kg (kilojoules per kilogram).
    • Example: For water, latent heat of vaporization is approximately 2257 kJ/kg at 100°C and 1 atm pressure.
  4. Reversible process
    • When vapor condenses back into liquid, the same amount of heat is released.

Importance of Latent Heat of Vaporization

  1. Steam Production
    • In boilers and power plants, heat is used to produce steam from water.
    • The energy added to convert water to steam is mainly the latent heat.
  2. Cooling Effect in Evaporation
    • When sweat evaporates from our body, it absorbs latent heat from the skin, giving a cooling effect.
  3. Air Conditioning and Refrigeration
    • Refrigerants absorb latent heat in the evaporator and release it in the condenser.
    • This cycle keeps spaces cool.
  4. Cloud and Rain Formation
    • Water vapor condenses in the atmosphere, releasing latent heat, which powers storms and weather systems.
  5. Boiling and Condensation Processes
    • Accurate understanding of latent heat is essential for boiler, turbine, and heat exchanger design.

Example:

Let’s say you want to turn 1 kg of water at 100°C into steam.

  • You need to supply approximately 2257 kJ of heat.
  • This heat doesn’t change the temperature but converts the water into vapor.

Once all the water turns into steam, if more heat is added, it increases the temperature of the steam, not the liquid.

Conclusion

The latent heat of vaporization is the hidden energy required to change a liquid into gas at a fixed temperature. It plays a critical role in many mechanical engineering systems, especially in power plants, refrigeration, weather processes, and human body cooling. Understanding this concept helps engineers in designing efficient energy systems that involve boiling, condensation, and heat transfer.