What is Latent Heat?

Latent heat is defined as heat energy that is absorbed or released during the transition phase of a substance. It can be either the transition of gas to a liquid or a liquid to a solid. The latent heat that is related to the total heat energy of a thermodynamic system is known as enthalpy.

The important point that should be considered regarding latent heat is that the temperature of th system remains constant. Latent heat is the work that is needed to repress the attractive forces that hold atoms and molecules together in a substance.

For example, suppose a solid substance is transforming into a liquid, it needs to absorb energy to push the molecules into a spacious or more fluid volume. Similarly, when a substance transforms from gas to a liquid, their density level also goes from higher to lower; whereas the substance that is required to release or lose some energy so that molecules do not come closer together. In short, the energy that is needed by the substance to either grease, melt, or boil is known as latent heat.

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Latent Heat Definition

The amount of heat radiated or absorbed during the transformation phase of a substance at constant temperature and pressure is known as latent heat.


Latent Heat Examples

The two Latent heat examples are given below.

  • The heat of freezing.

  • The heat of vaporization.

The heat of freezing is the amount of thermal energy discharged when a liquid freezes and the heat of vaporization is the amount of thermal energy that must be added up to transform the liquid into gas.


Latent Heat Formula

The latent heat formula is given as,

Q = M × L

In the above equation, 

Q is the amount of heat.

M is the mass of the substance.

The Latent Heat is represented by L.

The unit of latent heat is joules/Kg.

The above equation states that the heat ‘Q’ must either be subtracted or added up in the object of mass ‘M’ to transform the phases. 

The values of latent heat are variable and rely on the occurrence of the  transformation phase.

  • The latent heat of fusion implies the transformation of a liquid to a solid.

  • The latent heat of vaporization implies the change from a liquid to a gas.

  • The latent heat of sublimation implies the change from a solid to a gas.


Types of Latent Heat

There are three different types of latent heats, 

  1. Latent Heat of Fusion,

  2. Latent Heat of Vaporization,

  3. Latent Heat of Sublimation.


Let us know discuss the types of latent heat in detail.


Latent Heat of Fusion 

The latent heat of fusion or melting, of a solid, is the quantity of heat in joules required to transform a solid, at its melting point, to a liquid without any variation in temperature.

The latent heat of fusion of ice is 3.34 x 10⁵ joules per kilograms (or, 3.34 x 10⁵J/Kg).


Latent Heat of Vaporization

The term latent heat of vaporization is the amount of heat required for the transformation of a liquid at its boiling point to gas at a constant temperature. The latent heat of vaporization differs for various liquids.

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In the figure given above, the liquid is converting into vapour by heating.

Particles of water vapour at 100°C (373K) have more energy than liquid water at the same temperature. This is due to the assimilation of extra energy in the form of latent heat of vaporization.


Latent Heat of Sublimation

Some of the substances such as naphthalene get directly transformed from solid to gas when placed in the open atmosphere. The amount of heat needed by a substance to transform from solid to gaseous state or the amount of heat needed to be removed from a gaseous substance to transform into its solid state is known as the latent heat of sublimation.


Specific Latent Heat

It requires a certain amount of energy to transform the physical state of 1 kg of water from solid to liquid. This amount of energy is known as Specific Latent Heat( Lf ) of water.


The Definition:

“The amount of energy required per unit mass of ice to transform into water, without any change in the temperature is called the specific latent heat of water.”

Where:

  • ΔQ is the heat energy added.

  • M is the mass of a substance that undergoes a change of state.

  • There is no temperature parameter included in the equations as the temperature remains constant.

Note: There are two occasions when the change of state occurs, and both of these require different amounts of energy during the transition phase. Hence, there are two symbols:

  • Latent heat of fusion L- solid to liquid and vice-versa.

  • Latent heat of vaporization Lv - liquid to gas and vice-versa.


Specific Latent Heat of Vaporization of Water

Specific latent heat of vaporization of water is defined as the amount of heat required to transform a unit mass of water from the liquid state to the gaseous state without any variation in temperature.

The S.I. unit of specific latent heat of vaporization, Iv of a substance is Joules per kilogram (Jkg-1).

Q = Amount of thermal energy immersed or released.

M = Mass of the substance undergoing a change of state.

Lf = Specific latent heat of vaporization.


Solved Examples

1. Calculate the Latent Heat of 10 kg of Substance if the Amount of Heat for the Transition Phase is 300Kcal.

Solution:

Q = 300 Kcal

M = 10 Kg

Applying Latent Heat Formula,

L = Q/M

= 300/5

= 60 Kcal kg-1 

Hence, the latent heat will be 60 Kcal kg-1 g

FAQs (Frequently Asked Questions)

1. What are the Applications of Specific Latent Heat?

Answer: Some of the applications of specific latent heat are as follows. 


  • Quenching Fire by Using Boiling Water

Water that is boiled quickly becomes hot that is able to absorb a greater amount of latent heat from the fire.


  • Melting of Ice on the Road with the Help of Salt

As we know that the specific latent heat of fusion of salt is more than ice. Hence, the salt will require more heat energy when it is placed on the road having a thick layer of ice and absorbing energy from ice. Hence, ice will melt.


  • Cooling Drink by the Use of Cold Water and Ice

A glass of cold drink can be cooled easily by adding cold water or ice into it.

During the melting of ice, a large amount of latent heat is absorbed from the drink and this causes the drink to lose heat energy and its temperature lowers due to the melting of ice. Ice absorbs large quantities of latent heat during the process of melting.


  • Steaming Food

The specific latent heat of vaporization of water is immense. Plates filled with food are able to absorb heat from the steam. The condensation of steam at the base of the plate releases a large amount of heat and this enables food such as fish cake eggs to be steamed.

2. What is Latent Heat of Vaporization?

Answer: When any substance transforms its phase, the arrangement of its molecules varies but the temperature remains constant.

If the new arrangement has a greater amount of thermal energy, then the substance absorbs thermal energy from its environment in order to change the phase. If the new arrangement has a lower amount of thermal energy, then the substance releases thermal energy to its environment.

The latent heat of vaporization is a physical attribute of a substance. It is defined as the heat required to transform one mole of liquid at its boiling point under some standard atmospheric pressure. It is expressed in Kg/mole or KJ/KG.

When a material in the liquid state is given some energy, it transforms its phase from liquid to gas. Hence, the energy absorbed in this process is called the heat of vaporization. The heat of vaporization of water is about 2,260 KJ/ Kg which is equivalent to 40.8 KJ/ mol.