1. What Is Latent Heat?
Latent heat is the amount of heat absorbed or released by a substance during a change of state without any change in temperature. Even though heat is being added or removed, the thermometer shows no rise or fall until the entire state change is complete.
The heat goes into rearranging particles, not increasing their kinetic energy, which is why temperature stays constant.
2. Why Temperature Stays Constant
During melting, boiling, or any phase change, the energy supplied is used to change the bonding between particles. Since this energy does not increase particle motion, the temperature does not rise.
2.1. Energy Used to Break Bonds
When a solid melts or a liquid boils, particles need extra energy to break free from their fixed or close-packed positions. This is where the absorbed heat goes.
Because this energy does not increase kinetic energy, the temperature remains constant until the whole substance has changed state.
2.2. Energy Released During Reverse Processes
When a liquid freezes or a gas condenses, particles move closer. They release energy, which is why freezing or condensation can warm the surroundings even though the temperature of the substance itself remains constant.
3. Types of Latent Heat
There are two main types of latent heat depending on the phase change: fusion and vaporisation.
3.1. Latent Heat of Fusion
Latent heat of fusion is the heat absorbed when a solid changes into a liquid at its melting point without any change in temperature.
Example: Ice melting at 0°C absorbs heat but stays at 0°C until all the ice has melted.
3.2. Latent Heat of Vaporisation
Latent heat of vaporisation is the heat absorbed when a liquid changes into a gas at its boiling point.
Example: Water boils at 100°C but remains at this temperature until all the water turns into steam.
4. Latent Heat Formula
The heat required for a phase change is calculated using:
\( Q = mL \)
Where L is the latent heat of fusion or vaporisation, depending on the state change.
4.1. Meaning of Terms
- Q → Heat absorbed or released (J)
- m → Mass of the substance (kg)
- L → Latent heat (J/kg)
5. Role of Latent Heat in Nature and Daily Life
Latent heat explains many everyday observations and plays an important role in regulating temperature in the environment.
5.1. Cooling Effect of Evaporation
When sweat evaporates, it absorbs latent heat from the skin. This is why evaporation cools the body on hot days.
5.2. Melting of Ice in Drinks
Ice absorbs a large amount of heat while melting without increasing its temperature. This keeps drinks cold for longer.
5.3. Climate Moderation
Water in oceans absorbs and releases large amounts of latent heat during evaporation and condensation, helping to regulate temperatures across coastal regions.
6. Examples of Latent Heat in Action
- Steam condensing on a cooler surface releases heat and can make the surface warm.
- A pressure cooker boils water faster because higher pressure increases boiling point.
- Fog forms when water vapour condenses, releasing latent heat into the surrounding air.
- Snow absorbs latent heat while melting, which helps slow down rapid temperature rises in cold regions.