Latent heat is the energy transferred to or from a substance during a phase change without, under ideal equilibrium conditions, producing a change in the substance's temperature while the phase transition is occurring. The word latent historically refers to heat that seemed hidden because it did not immediately appear as a temperature increase. Modern thermodynamics generally describes the relevant quantity through enthalpy changes rather than treating heat as a substance stored inside matter.
Latent heat is associated with changes such as melting, freezing, boiling, condensation, sublimation and deposition. During melting, energy is absorbed as latent heat of fusion; during freezing, corresponding energy is released. During boiling or vaporisation, energy is absorbed as latent heat of vaporisation; during condensation, it is released. For sublimation, the relevant quantity is latent heat or enthalpy of sublimation, while deposition releases the corresponding energy. The key idea is that energy transfer changes the phase and microscopic arrangement rather than simply raising or lowering the average temperature.
The microscopic origin of latent heat lies in changes in molecular arrangement, intermolecular interactions and accessible configurations. When a solid melts, supplied energy helps transform an ordered structure into a liquid structure with greater molecular freedom. When a liquid vaporises, energy is required to separate molecules much more extensively and create a gas with far greater spatial freedom.
The temperature can remain nearly constant during an ideal phase transition because the energy entering the system is associated with the change in phase rather than increasing the average kinetic energy of the particles. Temperature is closely related to the average thermal energy associated with microscopic motion, while latent heat is connected to the energy difference between phases at the transition conditions. This distinction explains why supplying heat to boiling water can produce more steam without necessarily making the remaining liquid hotter. Conversely, when steam condenses, it releases substantial energy while the condensate can remain near the saturation temperature.
Latent heat is commonly quantified using the equation Q equals mL, where Q is the heat transferred, m is the mass undergoing the phase change and L is the specific latent heat for the relevant transition. Specific latent heat is measured in joules per kilogram.
For example, the specific latent heat of fusion of water is approximately 334 kilojoules per kilogram near its normal melting point, while the specific latent heat of vaporisation is approximately 2257 kilojoules per kilogram near its normal boiling point. These values vary with temperature and pressure, so they should be treated as representative values rather than universal constants. Molar latent heats are expressed in joules per mole and are often used in chemistry and thermodynamics. The sign of the heat transfer depends on the direction of the transition: melting and vaporisation generally require energy input, whereas freezing and condensation generally release energy. The magnitude for the reverse transition is equal under corresponding equilibrium conditions.
Latent heat is fundamental to the operation of refrigerators, heat pumps, steam engines and many other thermal systems. Refrigeration relies heavily on the large enthalpy changes associated with vaporisation and condensation of a working fluid. A refrigerant absorbs heat from a low temperature environment as it evaporates and releases heat to a higher temperature environment when it condenses, with compressors and expansion devices controlling pressure and temperature.
Steam turbines exploit the energy carried by high temperature and high pressure steam, while condensation can release substantial thermal energy to cooling systems. Heat pipes similarly use evaporation and condensation to transport heat efficiently over distances. In buildings and industrial systems, phase change materials can store thermal energy by melting and release it when freezing, providing temperature regulation. The usefulness of latent heat arises from the ability of a phase transition to transfer or store substantial amounts of energy at relatively stable temperatures.
Latent heat has major environmental significance. Water's large latent heat of vaporisation makes evaporation a powerful mechanism for transferring energy from Earth's surface into the atmosphere. When water evaporates from oceans, soil, vegetation and other surfaces, energy is absorbed. When water vapour later condenses into cloud droplets or precipitation, that energy is released into the atmosphere. This latent heat release is a major source of energy for atmospheric convection, thunderstorms, tropical cyclones and many other weather systems.
Freezing and melting of water likewise redistribute energy within the cryosphere and climate system. Snow and ice can absorb large quantities of heat during melting while remaining near their melting temperature, which can delay warming. The water cycle therefore functions partly as a planetary energy transport system in which latent heat moves energy over large distances through evaporation, atmospheric transport and condensation.
Latent heat should not be confused with sensible heat. Sensible heating or cooling changes the temperature of a material without necessarily changing its phase, while latent heat is associated with a phase transition. If water is heated from 20 degrees Celsius to 80 degrees Celsius, the supplied energy mainly produces a temperature increase and is therefore sensible heating.
If the water then boils at a fixed pressure, substantial additional energy is required to convert liquid into vapour while its temperature remains near the boiling temperature, representing latent heating. In real systems, phase transitions may occur over ranges rather than at a single temperature, particularly in mixtures, amorphous materials and complex biological systems. Some materials also undergo solid solid phase transitions that involve enthalpy changes without changing from solid to liquid or gas. Consequently, the broad thermodynamic concept is more general than the simple school level distinction between heating and phase change.
Latent heat also connects directly with thermodynamic state functions. At constant pressure, the heat associated with a reversible phase transition is related to the enthalpy difference between the phases, and the latent heat can therefore be described as a transition enthalpy under specified conditions. The Clausius Clapeyron relation connects the slope of a phase boundary in pressure temperature space with the latent heat and the volume change between phases.
This relationship explains why the pressure dependence of a boiling temperature contains information about the energy required for vaporisation. As a substance approaches its critical point, the distinction between liquid and gas becomes smaller, and the latent heat of vaporisation approaches zero. There is no ordinary latent heat associated with crossing a critical point because there is no first order liquid gas phase transition there. Similar considerations apply to other phase boundaries and demonstrate that latent heat is fundamentally a thermodynamic property rather than merely a quantity used in elementary heating calculations.
A common misconception is that latent heat is energy that disappears or becomes completely unavailable during a phase change. The energy remains part of the thermodynamic energy accounting of the system and surroundings, and it can be released again when the reverse transition occurs. Another misconception is that temperature always rises whenever heat is added. During an equilibrium phase transition, heat can be added while temperature remains nearly constant because the energy is changing the phase.
The reverse process releases the corresponding energy, often producing strong heating of the surroundings. Latent heat is therefore central to thermal buffering, phase change energy storage, atmospheric physics, refrigeration and industrial heat transfer. Its quantitative value depends on the substance and thermodynamic conditions, and its physical meaning is best understood as the enthalpy change associated with changing between phases rather than as a mysterious form of hidden heat.
