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Latent Heat Formula

Heat is an important component of phase changes. Latent heat is known as the heat required to convert a solid into a liquid or vapour phase. According to its phase, it has different names like the heat of condensation, the heat of vaporization, etc. Sometimes it is the amount of heat energy absorbed or released for a phase change. In this topic, we will deal with the concept of latent heat and latent heat formula with examples. Let us learn the concept!

Latent Heat Formula

Concept of Phase change:

Boiling the water is not as simple as just turning on the stove. As we apply heat to the water, the temperature increases until it reaches 100 degrees Celsius. The water will remain to stay at this temperature until all of the water changes from liquid to a gas.

During this process, we are continuing to add heat energy to the water, but still, the temperature does not increase. This extra energy giving rise to the concept of latent heat.

For better understanding, we first review the concept of phase changes. Phase change refers to a change in matter from one state to another. The most common phase changes are seen in the different states of water. Some examples are freezing liquid water to create ice or boiling liquid water to create a gas. As we can tell, heat plays a major role in changing the matter from one phase to another.

Definition of Latent Heat:

Normally, when we add or remove the heat energy from some object, the temperature of the object changes. But during the phase changes, the temperature of an object remains constant. This is because of the fact that energy is required for an object to change phases.

Therefore, Latent heat is the heat energy per mass unit required for a phase change to happen. If we consider the substances at a molecular level, gaseous molecules will have more vibration than the liquid molecules.

Thus, when we add heat to a liquid, we are actually causing the molecules to vibrate. The latent heat will be the energy required to change the molecular movement. Each substance has a unique latent heat value.

The Formula for Latent Heat:

L = \( \frac{Q}{ M} \)


Q = amount of heat

M = mass of the substance

This equation states that the heat Q that must be added or removed for an object of mass m to change phases. We denote Individual latent heat by L. The unit of latent heat is \(Jkg^{-1}.\)

The value of latent heat is variable. It depends on the nature of the phase change taking place:

  • The latent heat of fusion means the change from liquid to solid.
  • The latent heat of vaporization means from liquid to gas.
  • Latent heat of sublimation means the change from solid to gas.

Solved Examples 

Q.1: Compute the latent heat of 10 kg substance if the amount of heat for a phase change is 300 kcal.


Given parameters are,

Q = 300 kcal

M = 10 kg

Formula for latent heat is:

L = \(\frac{Q}{ M}\)

= \(\frac {300}{5}\)

=  \(60 KcalKg^{-1}g \)

Therefore latent heat will be 60 \(KcalKg{-1}\)

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