Latent Heat: Meaning, formula, Types

What is latent heat?

When substances transform from one state to another, they absorb or release energy. Part of this heat energy is utilized to overcome the forces of attraction between the molecules, and it is referred to as latent heat.

It’s evident that knowing how much energy it takes to change the state of a substance’s unit mass is useful. This is referred to as “specific latent heat,” and it is defined as follows:

The energy required to change the state of 1 kg of a substance is known as specific latent heat.

What should be kept in mind about latent heat is that in a closed system, the temperature of the system remains constant. It is only referring to the extra energy that is expelled or absorbed when the transition occurs. 

To help you understand, picture this scenario in mind: imagine a closed system with a substance present in it. Now we either increase or decrease the temperature of the system. When a sufficient amount of heat has been applied or removed, the substance then expels or absorbs a certain amount of energy which then allows it to transition from one state to another. This means that the temperature of the system remains constant while transferring heat only when the  transition is occurring.

Formula of Latent Heat

The formula for latent heat is,

L = Q/m

or

Q = m× L

where,

Q = amount of heat.

m = mass of the substance.

L = Latent heat of the substance.

Latent heat is generally represented in unit: joules/Kg

To transition the phases, the heat ‘Q’ must be subtracted or added to the mass ‘m’ in the object of mass ‘m’, according to the preceding equation. 

Types of Latent Heat

There are three types of latent heat

1. Latent heat of Vaporization
2. Latent heat of Fusion
3. Latent heat of Sublimation

We will now proceed to explain the different types of Latent Heat in detail

Latent heat of Vaporization 

The energy that is absorbed or expelled when a substance changes from a liquid state to a gaseous state, or vice versa. It is defined as follows:

The heat energy required to change 1 kilogram of a liquid at its boiling point to 1 kg of a gaseous state, as well as the heat energy released when 1 kg of vapor changes to 1 kg of liquid, is known as the specific latent heat of vaporization of a liquid.

Latent heat of fusion

The energy that is absorbed or expelled when a substance changes from a solid to a liquid state, or vice versa. It is defined as follows:

The heat energy required to change 1 kilogram of material in its solid state at its melting point to 1 kg of material in its liquid state, as well as the heat energy released when 1 kg of liquid changes to 1 kg of solid, is known as specific latent heat fusion.

Latent heat of Sublimation

While talking about latent heats, it is sublimation as a process is often overlooked which is erroneous. There is also a third type of latent heat which is expelled or absorbed when a substance undergoes sublimation.

Sublimation is a physical process in which a solid changes straight to a gaseous (vapor) state without first passing through a liquid stage. The amount of heat required to turn a unit quantity of solid into gas at a given temperature is known as the latent heat of sublimation. For example, the amount of heat necessary to sublimate ice into vapor at 0°C is calculated to be 2,838 kJ/kg, which is precisely the latent heat of sublimation of ice at 0°C. This process is important in the formation of ice and snow crystals in the atmosphere. This process is the polar opposite of deposition.

Sensible Heat

Sensible heat is often called a latent heat but it isn’t so. What is Sensible heat?

Sensible heat is not a constant-temperature scenario, nor does it include a phase transition. Heat transfer between matter and its surroundings is reflected as sensible heat. It is heat that can be “sensed” as a change in the temperature of an object.

Table with common substances and their Specific Latent Heat