Latent Heat

Latent heat refers to the amount of heat energy absorbed or released by a substance during a change of state (phase transition) without any change in its temperature. This phenomenon is fundamental in thermodynamics and plays a critical role in natural processes such as evaporation, condensation, freezing, and melting. The concept of latent heat is central to understanding how energy is exchanged during phase changes and is widely applied in meteorology, engineering, and physical sciences.

Background and Concept

The term latent is derived from the Latin word latere, meaning to lie hidden, signifying that this form of heat does not manifest as a temperature change. When a substance undergoes a phase transition—such as from solid to liquid or liquid to gas—the heat energy supplied or removed is utilised to break or form intermolecular bonds rather than increase kinetic energy.
For example, when ice melts at 0°C, it absorbs heat energy to overcome the molecular forces holding the solid structure together, converting into liquid water while maintaining the same temperature until the entire ice has melted. Similarly, during boiling, the supplied heat converts liquid water into vapour without changing the temperature beyond the boiling point (100°C under standard pressure).

Types of Latent Heat

There are two principal types of latent heat based on the kind of phase change involved:

1. Latent Heat of Fusion – The heat energy required to convert a unit mass of a solid into a liquid at constant temperature.
   • Example: The latent heat of fusion of ice is approximately 334 kJ/kg, meaning that 334 kilojoules of energy are needed to melt one kilogram of ice at 0°C.
2. Latent Heat of Vaporisation – The heat energy required to convert a unit mass of a liquid into vapour at constant temperature.
   • Example: The latent heat of vaporisation of water at 100°C is about 2260 kJ/kg, indicating that a large amount of energy is absorbed during the transformation from liquid to gas.

In addition to these, other forms such as latent heat of sublimation (solid to vapour transition) and latent heat of condensation (vapour to liquid transition) are also recognised, where heat is either absorbed or released depending on the direction of change.

Mathematical Expression

Latent heat can be mathematically expressed using the relation:
Q=mLQ = mLQ=mL
Where:

• QQQ = Heat absorbed or released (in joules)
• mmm = Mass of the substance (in kilograms)
• LLL = Specific latent heat (in joules per kilogram, J/kg)

This equation demonstrates that the total heat energy involved in a phase change depends directly on the mass of the substance and the type of phase transition.

Factors Affecting Latent Heat

Several factors influence the latent heat of a substance:

• Nature of the Substance: Different materials have different intermolecular bonding forces, which determine how much energy is needed to overcome or establish these bonds.
• Pressure: Changes in pressure can alter the latent heat. For instance, the latent heat of vaporisation of water decreases with increasing pressure.
• Temperature of Transition: The specific temperature at which phase change occurs affects the value of latent heat, as seen in substances under non-standard atmospheric conditions.

Applications in Everyday Life and Industry

Latent heat has numerous practical applications across science and engineering disciplines:

• Refrigeration and Air Conditioning: Cooling systems utilise the principle of latent heat of vaporisation and condensation to absorb and release heat efficiently. Refrigerants evaporate and condense cyclically to maintain desired temperatures.
• Meteorology: The formation of clouds, rainfall, and storms involves massive latent heat exchanges during water vapour condensation and freezing, influencing weather patterns and atmospheric circulation.
• Thermal Energy Storage: Phase change materials (PCMs) use latent heat to store and release thermal energy, making them useful in building temperature regulation and solar energy systems.
• Steam Engines and Power Plants: The generation of steam and its subsequent condensation to water in turbines depend on the latent heat of vaporisation and condensation, forming the basis of thermal power generation.
• Food Preservation: Processes like freeze-drying and cryogenic freezing rely on latent heat removal to preserve perishable items without damaging their structure.

Latent Heat in Natural Phenomena

Latent heat plays a vital role in maintaining climatic and environmental balance. During evaporation, water absorbs latent heat from its surroundings, causing cooling effects that are evident in processes like perspiration and transpiration in plants. Conversely, condensation releases latent heat into the atmosphere, contributing to energy transfer mechanisms within weather systems.
In the hydrological cycle, latent heat facilitates the continuous transformation of water between its various states—solid, liquid, and gas—ensuring the dynamic equilibrium of the Earth’s climate system.

Measurement and Experimental Determination

Latent heat can be determined experimentally through calorimetry. The principle involves measuring the heat exchange between a phase-changing substance and its surroundings. For example, to measure the latent heat of fusion of ice, a known mass of ice is melted in a calorimeter containing water, and the resulting temperature change helps compute the absorbed heat.
Accurate determination of latent heat values is essential in designing efficient heat exchangers, predicting weather models, and understanding energy transfer in both natural and industrial processes.

Significance in Thermodynamics

In thermodynamics, latent heat represents a key aspect of energy conservation during state changes. It differentiates between sensible heat (which changes temperature) and hidden or latent heat (which changes phase). The concept underpins the first law of thermodynamics, ensuring that energy transformations are accounted for even when temperature remains constant.