Latent heat

Latent heat is energy transferred while matter changes phase at an essentially constant temperature. During melting or boiling, the supplied energy does not primarily increase the average translational speed of particles; it helps alter their bonding and arrangement. During freezing or condensation, the corresponding energy is released to the surroundings.¹

The word “latent” means hidden because the energy transfer is not immediately visible as a temperature rise. A thermometer can remain at the melting point while ice absorbs heat and becomes liquid, or at the boiling point while liquid water becomes vapor. The temperature remains constant only under conditions where the phases coexist in equilibrium; pressure and composition affect the transition temperature and the associated energy.²

The heat involved is calculated as Q = mL, where Q is transferred heat, m is mass, and L is specific latent heat. Specific latent heat is expressed in joules per kilogram, whereas total latent heat is an amount of energy in joules. Its value depends on the substance, the particular phase transition, and usually the pressure or temperature at which the transition occurs.¹

• Latent heat of fusion accompanies melting and freezing.
• Latent heat of vaporization accompanies boiling, evaporation, and condensation.
• Latent heat of sublimation accompanies direct solid–gas transitions, such as sublimation and deposition.

For water at standard atmospheric pressure, melting ice requires about 334 kilojoules per kilogram, while vaporizing liquid water requires about 2,260 kilojoules per kilogram; tabulated values vary with temperature and pressure.¹³

Latent heat is a macroscopic expression of changes in a material’s internal energy during a first-order phase transition. At equilibrium, heat can enter or leave while temperature remains fixed because the material’s phase proportions change instead of its temperature changing.²

The magnitude is not universal even for one substance: the latent heat of vaporization generally decreases as the critical point is approached, where liquid and vapor become indistinguishable. Pressure also matters, because it changes the equilibrium conditions and the volume difference between phases. The Clausius–Clapeyron relation connects the slope of a phase boundary to latent heat, temperature, and the volume change between phases. Near ordinary transitions, calorimetry measures latent heat by tracking energy and phase fraction rather than relying only on a temperature change.³⁴

Latent heat governs major transfers of energy in weather, engineering, and natural environments. Evaporation removes heat from a surface, while condensation releases it; this powers cloud formation and helps drive atmospheric convection. Refrigerators and heat pumps exploit evaporation and condensation of a working fluid, and thermal-energy storage can use melting materials to absorb heat near a controlled temperature.

Several edge cases complicate the simple textbook picture. Freezing water can remain liquid below its equilibrium freezing point through supercooling, then release latent heat abruptly when crystallization begins. Evaporation can occur below the normal boiling point because it is a surface process, whereas boiling requires vapor bubbles to form throughout the liquid. In mixtures, phase change often occurs across a temperature range rather than at one fixed temperature. Condensation and deposition release latent heat, so “latent” does not mean energy is absent; it means the transfer is concealed by the phase transition rather than recorded as sensible heating or cooling.

Specific latent heat

The energy required or released per unit mass during a specified phase change, measured in joules per kilogram.

Sensible heat

Heat transfer that changes temperature without changing the material’s phase.

Phase equilibrium

A condition in which coexisting phases remain stable because their opposing transformations balance.

Critical point

The endpoint of a liquid–vapor equilibrium boundary, beyond which distinct liquid and gas phases do not exist.