Latent Heat

Heat is one of the key factors catalysing the change of the physical state of matter. When we heat ice, it melts into water, and when we boil water, it turns into steam. Interestingly, during these changes of state, the temperature of the substance remains constant, even though heat is being supplied. This “hidden” heat is known as latent heat. The word ‘Latent’ indicates the fact that the energy is absorbed or released without causing a change in temperature.

About Latent Heat

Latent heat is the amount of heat absorbed or released by a substance during a change of state without any change in its temperature. These states can be melting, freezing, boiling or condensation. The phenomenon happens because the supplied energy breaks or forms intermolecular bonds rather than increasing the kinetic energy of molecules.

For example, when ice melts at Celsius, all the heat energy supplied is used to break hydrogen bonds in solid water. The temperature of the water begins to rise only after the ice has melted completely.

Formula of Latent Heat

Latent heat is measured in joules per kilogram (J/kg) and is expressed mathematically as:

Q = mL,

Where

Q is heat absorbed or released (in Joules)
M is the mass of the substance (in kilograms)
L is latent heat (in J/kg)

Difference Between Latent Heat and Specific Latent Heat

The concepts are confusing due to their similarity. Hence, here are the differences between the two:

Parameter	Latent Heat	Specific Latent Heat
Definition	Total amount of heat absorbed or released during a change of state, without temperature change.	Heat absorbed or released per unit mass (1 kg) of a substance during a change of state, without temperature change.
Formula	Q = mL	L = Q/m
Depends on	Both the mass of the substance and its nature.	Only on the nature of the substance, not on mass.
Units	Joules (J)	Joules per kilogram (J/kg)

Types of Latent Heat

There are two primary types of latent heat, depending on whether the substance is melting/freezing or vaporizing/condensing.

1. Latent Heat of Fusion

It is the amount of heat required to change a unit mass of a solid into liquid, or vice versa, at constant temperature.
Example: Ice at 0 °C requires about 334 kJ/kg of energy to melt into water without changing its temperature.

2. Latent Heat of Vaporization

It is the heat required to change a unit mass of a liquid into a gas, or a gas back into a liquid, at constant temperature.
Example: Water at 100 °C requires about 2260 kJ/kg to turn into steam.

Factors Affecting Latent Heat

Latent heat is the energy required or released when a substance changes its state. Here is what it depends upon:

Mass of the Substance: Latent heat is directly proportional to the mass of the substance. Hence, the greater the mass, the more total heat is required for the state change. Example: Melting 2 kg of ice requires double the energy needed for 1 kg of ice.

Nature of the Substance: Different substances have different molecular bonds. Latent heat is directly proportional to the strength of the bond. Stronger bonds require more energy to break. For instance, water has a higher latent heat compared to many liquids because of its strong hydrogen bonding. Type of Phase Change: Fusion (solid ↔ liquid) usually requires less energy than vaporization (liquid ↔ gas). This is why the latent heat of vaporization of water (2260 kJ/kg) is much greater than its latent heat of fusion (334 kJ/kg).

Temperature and Pressure Conditions: Latent heat values can change slightly depending on pressure. For example, at higher altitudes (low pressure), water boils at lower temperatures and requires slightly different latent heat.

Practical Examples of Latent Heat

Concepts of latent heat are witnessed in everyday life. Here are some practical examples:

Melting of Ice

When ice melts, it absorbs the latent heat of fusion from the surroundings. This is the reason why ice feels cold because the heat is drawn from your skin and the air, causing a cooling effect.

Boiling of Water

While boiling water at 100 °C, no temperature increase occurs until all the liquid turns to steam. This is due to the latent heat of vaporization.

Sweating and Human Cooling

Sweat absorbs heat from the body and evaporates, using the latent heat of vaporization. This process cools the body naturally and is essential for temperature regulation.

Summary

Latent heat is the heat absorbed or released by a substance during a change of state without a change in temperature. The energy is used to break or form intermolecular bonds. There are two types of latent heat: latent heat of fusion and latent heat of vaporization. Mass and nature of substance, type of phase change and temperature/pressure influence latent heat.