Short Answer

Latent heat is the amount of heat energy absorbed or released by a substance during a phase transition without a change in temperature. It is the energy required to overcome or form intermolecular forces when matter changes its state.

For example, during melting, heat is absorbed to break bonds in a solid, while during freezing, the same amount of heat is released to form bonds in the liquid. Latent heat explains why temperature remains constant during phase changes.

Detailed Explanation

Latent Heat

Latent heat is a key concept in thermodynamics and chemistry, describing the heat energy involved in phase transitions. The term “latent” means hidden, as this energy does not raise the temperature of the substance but is used to change the physical state.

This energy is needed to overcome or form intermolecular forces in a substance when it changes from solid to liquid, liquid to gas, or solid to gas. The temperature of the substance remains constant during the phase change, despite the absorption or release of energy.

1. Types of Latent Heat

Latent heat can be classified based on the type of phase transition:

1. Latent Heat of Fusion

• Energy absorbed or released when a solid changes to liquid or vice versa.
• Example: Ice melting into water absorbs latent heat; water freezing releases the same amount.

1. Latent Heat of Vaporization

• Energy absorbed or released during liquid-to-gas or gas-to-liquid transition.
• Example: Water boiling absorbs latent heat; condensation of steam releases heat.

1. Latent Heat of Sublimation

• Energy absorbed or released during solid-to-gas or gas-to-solid transition.
• Example: Dry ice subliming absorbs latent heat; deposition of water vapor as frost releases heat.

2. Characteristics of Latent Heat

• No Temperature Change: The substance stays at the phase transition temperature until the process completes.
• Energy Transfer: It is either absorbed (endothermic) or released (exothermic) depending on the direction of the phase change.
• Dependent on Substance: Different substances have different latent heats depending on intermolecular forces.
• Measured per Mass: Usually expressed in Joules per gram (J/g) or Joules per kilogram (J/kg).

For example:

• Latent heat of fusion of ice = 334 J/g
• Latent heat of vaporization of water = 2260 J/g

This shows how much energy is needed or released for each gram of water during phase changes.

3. Role of Latent Heat in Daily Life
4. Melting and Freezing

• Ice in drinks absorbs heat from the drink during melting, cooling it.
• Freezing water releases heat to surroundings, helping in natural freezing processes.

1. Boiling and Condensation

• Steam releases latent heat when condensing, warming surfaces.
• Boiling water absorbs latent heat without temperature rise, essential for cooking.

1. Weather and Climate

• Condensation of water vapor in clouds releases latent heat, driving wind and storms.
• Evaporation absorbs latent heat, cooling oceans and surfaces.

1. Industrial Applications

• Freeze-drying and refrigeration rely on energy changes due to latent heat.
• Steam engines and turbines utilize latent heat during phase transitions.

4. Latent Heat and Intermolecular Forces

Latent heat is closely linked to intermolecular forces:

• Stronger forces → Higher latent heat:
  Water has strong hydrogen bonds, so melting and vaporization require more energy.
• Weaker forces → Lower latent heat:
  Substances like iodine or naphthalene have weaker forces, requiring less energy for phase changes.

This explains why different substances require different amounts of heat for the same phase transition.

5. Measurement of Latent Heat

Latent heat is measured using calorimetry:

• A known mass of substance is subjected to heat at constant temperature.
• The energy absorbed or released is calculated using the formula:

Where:

•  = heat absorbed/released
•  = mass of substance
•  = latent heat per unit mass

This simple relationship helps determine energy requirements in industrial and laboratory processes.

Conclusion

Latent heat is the hidden energy absorbed or released during a phase transition without a temperature change. It is essential for melting, boiling, condensation, sublimation, and deposition. Understanding latent heat helps explain everyday phenomena, natural processes, and industrial applications. Its value depends on the type of substance and the strength of intermolecular forces.