What is the primary difference between latent heat and specific heat?

The primary difference lies in their effect on temperature. Specific heat capacity ($c$) is the amount of heat required to raise the temperature of a unit mass of a substance by one degree Celsius (or Kelvin) without changing its phase. It's associated with a change in temperature ($Q = mcDelta T$). Latent heat ($L$), on the other hand, is the heat absorbed or released during a phase change (like melting or boiling) at a constant temperature. It's associated with a change in state, not temperature ($Q = mL$). Specific heat changes kinetic energy, while latent heat changes potential energy related to molecular arrangement.

Why does temperature remain constant during a phase change, even though heat is being continuously supplied?

During a phase change, the heat energy supplied is not used to increase the average kinetic energy of the molecules, which is directly related to temperature. Instead, this energy is entirely utilized to overcome or establish the intermolecular forces that hold the molecules in a particular phase. For example, during melting, heat energy breaks the bonds in the solid lattice. During boiling, it separates molecules into a gaseous state. This energy is stored as potential energy within the substance, allowing the phase transition to occur without a corresponding increase in temperature.

Why is the latent heat of vaporization usually much higher than the latent heat of fusion for most substances?

The latent heat of vaporization ($L_v$) is typically much higher than the latent heat of fusion ($L_f$) because significantly more energy is required to completely separate molecules from the liquid state into a gaseous state compared to merely loosening their bonds from a solid to a liquid state. In vaporization, molecules must overcome almost all intermolecular attractive forces to move freely and independently as a gas, requiring a substantial energy input. In fusion, only enough energy is needed to disrupt the rigid solid structure, allowing molecules to slide past each other in the liquid phase, which requires less energy.

Can latent heat be released as well as absorbed?

Yes, absolutely. Latent heat is absorbed when a substance changes from a lower energy phase to a higher energy phase (e.g., solid to liquid - melting, or liquid to gas - vaporization). Conversely, latent heat is released when a substance changes from a higher energy phase to a lower energy phase (e.g., gas to liquid - condensation, or liquid to solid - freezing). For instance, when steam condenses on your skin, it releases its latent heat of vaporization, which is why steam burns are often more severe than hot water burns at the same temperature.

How is latent heat relevant in everyday life or medical contexts?

Latent heat is crucial in many everyday phenomena and has significant medical relevance. For instance, sweating cools the human body because the evaporation of sweat (liquid to gas) absorbs latent heat of vaporization from the skin. In medical procedures, understanding latent heat is vital for cryotherapy (using cold to treat tissues, where ice absorbs latent heat) or in understanding how fever breaks (sweating and evaporation). Refrigeration and air conditioning systems also fundamentally rely on refrigerants absorbing and releasing latent heat during their phase changes to cool spaces.

Latent Heat

Physics

NEET UG

Version 1Updated 24 Mar 2026

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Latent heat, a fundamental concept in thermodynamics, refers to the heat energy absorbed or released by a substance during a phase transition (e.g., melting, freezing, boiling, condensation) at a constant temperature and pressure. Unlike specific heat, which causes a change in temperature, latent heat is entirely utilized to alter the molecular arrangement and intermolecular forces within the subs…

Quick Summary

Latent heat is the 'hidden' heat energy absorbed or released by a substance during a phase transition (like melting, freezing, boiling, or condensation) without any change in its temperature. This energy is used to change the potential energy associated with intermolecular forces, rather than increasing the kinetic energy of molecules.

The two main types are latent heat of fusion ( $L_f$ ), for solid-liquid transitions, and latent heat of vaporization ( $L_v$ ), for liquid-gas transitions. The formula for calculating heat involved in a phase change is $Q = mL$ , where $m$ is mass and $L$ is the specific latent heat.

For water, $L_f = 3.34 \times 10^5,\text{J/kg}$ and $L_v = 2.26 \times 10^6,\text{J/kg}$ . Latent heat is vital for understanding heating curves, calorimetry problems, and various natural and technological processes like sweating, refrigeration, and weather phenomena.

It's a key concept for NEET, often tested in combination with specific heat calculations.

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Key Concepts

Calculating Heat for Melting Ice

When ice melts, it absorbs latent heat of fusion. The temperature remains $0^circ\text{C}$ until all the ice…

Calculating Heat for Vaporizing Water

When water boils, it absorbs latent heat of vaporization. The temperature remains $100^circ\text{C}$ (at…

Combined Heating and Phase Change

Many NEET problems involve both temperature changes and phase changes. To solve these, you must calculate the…

Latent Heat ($L$) — Heat absorbed/released during phase change at constant temperature.

Formula —

Q = mL

Latent Heat of Fusion ($L_f$) — Solid

ightleftharpoons

Liquid. For water:

L_f = 3.34 \times 10^5,\text{J/kg}

(

80,\text{cal/g}

Latent Heat of Vaporization ($L_v$) — Liquid

ightleftharpoons

Gas. For water:

L_v = 2.26 \times 10^6,\text{J/kg}

(

540,\text{cal/g}

Key Principle — Energy changes potential energy (intermolecular forces), not kinetic energy (temperature).

Heating Curve — Flat plateaus indicate phase changes (

Q=mL

), sloped sections indicate temperature changes (

Q=mcDelta T

L.A.T.E.N.T. H.E.A.T. Liquid to gas, Absorbs heat. Temperature Equal, No change at all. Transforms state, Hidden energy's call. Energy for bonds, Alters molecular space. Two types: Fusion and Vaporization's embrace.