What is latent heat?

Short Answer

Latent heat is the amount of heat energy required to change the state of a substance without changing its temperature. It is used either to convert a solid into a liquid or a liquid into a gas, or the reverse processes. During this change, the temperature remains constant even though heat is being absorbed or released.

Examples include heat absorbed when ice melts into water and heat released when steam condenses into water. Latent heat is important for understanding melting, boiling, freezing, and condensation.

Detailed Explanation :

Latent Heat

Latent heat is a special kind of heat energy involved in changing the state of matter. The word “latent” means “hidden,” because this heat does not raise the temperature of the substance. Instead, it is used to change the arrangement of particles from one state to another. When a substance changes its state—for example, from solid to liquid or from liquid to gas—it either absorbs or releases latent heat.

Latent heat is essential for many natural and daily-life processes such as melting ice, boiling water, evaporating sweat, freezing liquids, and cloud formation. Without latent heat, matter would not be able to change states smoothly.

Types of Latent Heat and How It Works

Latent heat plays a major role in state changes by helping particles overcome or strengthen intermolecular forces.

  1. Latent Heat of Fusion

This is the heat energy required to convert a solid into a liquid at its melting point without a change in temperature.

How it works:

  • The solid absorbs heat.
  • Its particles vibrate faster.
  • The forces holding particles in fixed positions weaken.
  • The solid melts into a liquid.

Even though heat is supplied, the temperature does not rise until the entire solid has melted.

Example:
Ice melts at 0°C, and during melting, the temperature stays at 0°C until all ice becomes water.

  1. Latent Heat of Vaporization

This is the heat energy required to convert a liquid into a gas at its boiling point without changing its temperature.

How it works:

  • The liquid absorbs a large amount of heat.
  • Particles gain enough energy to overcome forces of attraction.
  • Bubbles of vapor form inside the liquid.
  • The liquid turns into gas at constant temperature.

Example:
Water boils at 100°C, and the temperature remains 100°C until all water becomes steam.

Why Is Temperature Constant During Latent Heat?

During a change of state, heat energy does not increase the temperature. Instead, the energy is used to:

  • Break the bonds between particles (in melting or boiling)
  • Form new bonds between particles (in freezing or condensation)

Because the heat is used for structural changes, not for increasing particle movement, the temperature remains the same.

Latent Heat During Cooling

Latent heat is also released when a substance changes from a higher-energy state to a lower-energy state.

  1. Freezing (Liquid → Solid)

When a liquid freezes, particles lose energy and form a fixed structure. Latent heat is released into the surroundings.

  1. Condensation (Gas → Liquid)

When vapor condenses, it releases large amounts of latent heat.

Example:
Steam burns are more severe than hot water burns because steam releases latent heat while condensing on the skin.

Importance of Latent Heat

  1. Weather and Climate

Latent heat plays a major role in cloud formation, rainfall, and wind movement.

  • Water vapor rises and cools.
  • It condenses and releases latent heat.
  • This heats the air and affects weather patterns.
  1. Natural Cooling Process

Evaporation of sweat uses latent heat of vaporization, helping the body stay cool.

  1. Food and Cooking

Latent heat affects:

  • Boiling of water
  • Steam cooking
  • Freezing in refrigerators
  1. Industrial Uses

Latent heat is used in:

  • Refrigerators and air conditioners
  • Ice-making machines
  • Distillation processes
  • Thermal energy storage systems
  1. Identification of Substances

Different substances have different latent heats. This helps in studying their physical properties.

Examples Showing Latent Heat in Daily Life

  • Ice cube melting in a drink
  • Water boiling in a kettle
  • Dew forming on grass
  • Wet clothes drying
  • Clouds forming in the sky
  • Freezing of ice cream

All these involve latent heat being absorbed or released.

Latent Heat Compared to Sensible Heat

  • Latent Heat: Causes change of state without changing temperature.
  • Sensible Heat: Raises or lowers temperature without changing state.

Latent heat is “hidden” because we do not see a temperature change even though energy is being transferred.

Conclusion

Latent heat is the heat energy required to change the state of a substance without changing its temperature. It includes latent heat of fusion and latent heat of vaporization. During melting and boiling, heat is absorbed, while freezing and condensation release heat. Latent heat plays a key role in natural processes, weather, cooking, cooling, and many scientific applications. Without latent heat, matter would not be able to shift smoothly between solid, liquid, and gas states.