Latent Heat of Water: Definition, Formula, Fusion, Vaporisation and Everyday Examples

Latent heat of water is the heat energy absorbed or released when water changes its state without any change in temperature. Have you ever thought why ice remains at 0°C while melting or why boiling water stays at 100°C even though heat is continuously supplied? This happens because the heat is used to change the state of water rather than increase its temperature. Interesting!

We already know that water exists in three different states: solid (ice), liquid (water) and gas (steam). As it changes from one state to another state, it absorbs or releases latent heat while its temperature remains the same until the entire phase change is complete. 

This article is a perfect guide to learn what latent heat of water is, its mathematical formula, the latent heat of fusion of water, the latent heat of vaporisation of water and their everyday examples.

Table of Contents

• What is Latent Heat of Water
• Mathematical Formula of Latent Heat of Water
• Why does Temperature Remain Constant During a Phase Change
• Different Types of Latent Heat of Water
• Factors Affecting Latent Heat of Water
• Everyday Examples of Latent Heat of Water
• Frequently Asked Questions about Latent Heat of Water

What is Latent Heat of Water

Latent heat of water is nothing but the amount of heat energy absorbed or released when water changes its state without any change in temperature. During a phase change, the energy supplied does not increase or decrease the temperature. Instead of this, it is used to break or form the intermolecular forces between water molecules.

For better understanding, consider an example. When ice melts into water at 0°C, or when water boils to form steam at 100°C, the temperature remains constant until the entire phase change is complete. And ,the heat involved in these processes is called latent heat.

So, in simple words, latent heat is:

The amount of heat energy absorbed or released by a substance during a change of state without any change in its temperature.

The two main types of latent heat of water are,

• Latent Heat of Fusion: The heat required to convert ice into water or released when water freezes into ice at 0°C without changing the temperature.
• Latent Heat of Vaporisation: The heat required to convert water into steam or released when steam condenses into water at 100°C without changing the temperature.

Do you know? These concepts help explain why ice takes time to melt and why boiling water remains at 100°C until all the water changes into steam. Interesting, isn't it? 

Mathematical Formula of Latent Heat of Water

The amount of heat absorbed or released during a change of state can be calculated using the latent heat formula. Here is the mathematical formula, 

Q=mL

Where, 

Symbol	Meaning	SI Unit
Q	Heat absorbed or released during the phase change	Joule (J)
m	Mass of the substance	Kilogram (kg)
L	Specific latent heat of the substance	J/kg

This formula shows that the heat required for a phase change depends on the mass of the substance and its latent heat.

Important Values for Water

Process	Latent Heat Value
Latent heat of fusion of water	3.34 × 10⁵ J/kg (80 cal/g)
Latent heat of vaporisation of water	2.26 × 10⁶ J/kg (540 cal/g)

Why does Temperature Remain Constant During a Phase Change

When a substance changes from one state to another, such as ice melting into water or water boiling into steam, its temperature remains constant even though heat is continuously supplied.

And do you know this happens because the heat energy is not used to increase the temperature of the substance? Instead, it is used to overcome the intermolecular forces that hold the particles together. This energy changes the state (phase) of the substance and is called latent heat.

You will be amazed to know that, once the entire substance has completed the phase change, any additional heat supplied increases the kinetic energy of the particles and the temperature starts rising again.

Example

• When ice at 0°C is heated, its temperature remains 0°C until all the ice melts into water.
• Similarly, when water at 100°C is heated, its temperature remains 100°C until all the water changes into steam.

And this is the reason why temperature remains constant during a phase change, even though heat energy is continuously absorbed or released.

Different Types of Latent Heat of Water

Water undergoes two main types of latent heat during a change of state. In both cases, heat is absorbed or released without any change in temperature.

Latent Heat of Fusion of Water

The latent heat of fusion of water is the amount of heat required to convert 1 kg of ice at 0°C into 1 kg of water at 0°C without changing its temperature.

• Value: 3.34 × 10⁵ J/kg (334 kJ/kg)
• During melting, water absorbs heat.
• During freezing, the same amount of heat is released.

Example: Ice cubes melting in a drink absorb latent heat while remaining at 0°C until they completely melt.

Latent Heat of Vaporisation of Water

The latent heat of vaporisation of water is the amount of heat required to convert 1 kg of water at 100°C into 1 kg of steam at 100°C without changing its temperature.

• Value: 2.26 × 10⁶ J/kg (2260 kJ/kg)
• During boiling, water absorbs heat.
• During condensation, the same amount of heat is released.

Example: Water boiling in a kettle remains at 100°C until it completely changes into steam.

Latent Heat of Condensation

The latent heat of condensation is the heat released when water vapour changes into liquid water without any change in temperature.

• The amount of heat released is equal to the latent heat of vaporisation.
• Value: 2.26 × 10⁶ J/kg (2260 kJ/kg)

Example: Water droplets forming on the outside of a cold glass are produced when water vapour in the air condenses and releases latent heat.

Factors Affecting Latent Heat of Water

The amount of latent heat required for water depends on the type of phase change taking place. During a phase change, the temperature remains constant because the supplied or released heat is used to break or form intermolecular bonds instead of changing the temperature.

The following points explain the important factors that are related to the latent heat of water,

• Latent Heat of Vaporisation: When water at 100°C changes into steam at 100°C, it absorbs about 540 cal/g (approximately 2260 kJ/kg) of heat without any rise in temperature.
• Condensation: When steam at 100°C changes back into water at 100°C, the same amount of heat (540 cal/g) is released to the surroundings.
• Latent Heat of Fusion: When ice at 0°C melts into water at 0°C, it absorbs about 80 cal/g (approximately 334 kJ/kg) of heat without changing its temperature.
• Freezing: When water at 0°C freezes into ice at 0°C, the same amount of heat (80 cal/g) is released during the freezing process.
• Strength of Intermolecular Forces: Water has strong hydrogen bonds between its molecules. A large amount of energy is required to break these bonds during melting or vaporisation, which is why water has a high latent heat.
• Nature of the Phase Change: Different phase changes require different amounts of energy. Converting water into steam requires much more energy than melting ice because the molecules must separate completely during vaporisation.

Everyday Examples of Latent Heat of Water

Latent heat of water can be observed in many everyday situations where water changes its state without changing its temperature. Some common examples are,

• Ice Melting: When ice melts at 0°C, it absorbs latent heat of fusion from its surroundings. The temperature remains 0°C until all the ice has melted.
• Water Boiling: When water boils at 100°C, it absorbs latent heat of vaporisation and changes into steam. The temperature stays at 100°C until all the water has evaporated.
• Steam Condensing: When steam comes in contact with a cooler surface, it changes back into water by releasing latent heat. This is why water droplets form on the lid of a boiling pot.
• Formation of Dew: During cool nights, water vapour in the air condenses into tiny water droplets on grass and leaves. During this process, latent heat is released to the surroundings.
• Sweating Keeps the Body Cool: When sweat evaporates from your skin, it absorbs latent heat from your body. This removes heat from the skin and helps maintain body temperature.
• Cloud Formation and Rainfall: Water from oceans, rivers, and lakes evaporates by absorbing latent heat. As the water vapour rises and condenses into clouds, it releases latent heat, which plays an important role in weather and rainfall.

Till now, we have seen that the latent heat of water explains why water can change from ice to liquid or from liquid to steam without any change in temperature. During a phase change, the heat supplied or released is used to break or form intermolecular forces rather than increasing or decreasing the temperature.