Latent heat is the heat released or absorbed by a body or a thermodynamic system during a process that occurs without a change in temperature. A typical example is a change of state of matter, meaning a phase transition such as the melting of ice or the boiling of water.^[1][2] The term was introduced around 1762 by Joseph Black. It is derived from the Latin latere (to lie hidden). Black used the term in the context of calorimetry when referring to the heat transferred that caused a change of volume while the thermodynamic system was held at constant temperature.

In contrast to latent heat, an energy is called a sensible energy or heat, when it causes processes that do result in a change of the temperature of the system.

Thermodynamics
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Usage[link]

Two of the more common forms of latent heat (or enthalpies or energies) encountered are latent heat of fusion (melting or freezing) and latent heat of vaporization (boiling or condensing). These names describe the direction of energy flow when changing from one phase to the next: from solid to liquid, and to gas.

In both cases, the change is endothermic, meaning that the system absorbs energy on going from solid to liquid to gas. The change is exothermic (the process releases energy) for the opposite direction. For example, in the atmosphere, when a molecule of water evaporates from the surface of any body of water, energy is transported by the water molecule into a lower temperature air parcel that contains less water vapour than its surroundings. Because energy is needed to overcome the molecular forces of attraction between water particles, the process of transition from a parcel of water to a parcel of vapor requires the input of energy causing a drop in temperature in its surroundings. If the water vapor condenses back to a liquid or solid phase onto a surface, the latent energy absorbed during evaporation is released as sensible heat onto the surface. The large value of the enthalpy of condensation of water vapour is the reason that steam is a far more effective heating medium than boiling water, and is more hazardous.

The terms sensible heat and latent heat are not special forms of energy; instead they characterize the same form of energy, heat, in terms of their effect on a material or a thermodynamic system. Heat is thermal energy in the process of transfer between a system and its surroundings or between two systems with a different temperature.

Both sensible and latent heats are observed in many processes while transporting energy in nature. Latent heat is associated with the phase changes of atmospheric water vapour, mostly vaporization and condensation, whereas sensible heat is energy transferred that affects the temperature of the atmosphere.

The original usage of the term, as introduced by Black, was applied to systems that were intentionally held at constant temperature. Such usage referred to latent heat of expansion and several other related latent heats. These latent heats are defined independently of the conceptual framework of thermodynamics.^[3]

When a body is heated at constant temperature by thermal radiation in a microwave field for example, it may expand by an amount described by its latent heat with respect to volume or latent heat of expansion, or increase its pressure by an amount described by its latent heat with respect to pressure.^[4]

Meteorology[link]

In meteorology, latent heat flux is the flux of heat from the Earth's surface to the atmosphere that is associated with evaporation or transpiration of water at the surface and subsequent condensation of water vapor in the troposphere. It is an important component of Earth's surface energy budget. Latent heat flux has been commonly measured with the Bowen ratio technique, or more recently since the mid-1900s by the eddy covariance method.

History[link]

The English word latent comes from Latin latēns, meaning lying hid.^[5][6] The term latent heat was introduced into calorimetry around 1750 by Joseph Black when studying system changes, such as of volume and pressure, when the thermodynamic system was held at constant temperature in a thermal bath. James Prescott Joule characterized latent energy as the energy of interaction in a given configuration of particles, i.e. a form of potential energy, and the sensible heat as an energy that was indicated by the thermometer,^[7] relating the latter to thermal energy.

Specific latent heat[link]

A specific latent heat (L) expresses the amount of energy in form of heat (Q) required to completely affect a phase change of a unit of mass (m), usually 1kg, of a substance as an intensive property:

Failed to parse (Missing texvc executable; please see math/README to configure.): L = \frac {Q}{m}

Intensive properties are material characteristics and are not dependent on the size or extent of the sample. Commonly quoted and tabulated in the literature are the specific latent heat of fusion and the specific latent heat of vaporization for many substances.

From this definition, the latent heat for a given mass of a substance is calculated by

Failed to parse (Missing texvc executable; please see math/README to configure.): Q = {m} {L}

where:

Q is the amount of energy released or absorbed during the change of phase of the substance (in kJ or in BTU),

m is the mass of the substance (in kg or in lb), and

L is the specific latent heat for a particular substance (kJ-kg_m⁻¹ or in BTU-lb_m⁻¹), either L_f for fusion, or L_v for vaporization.

Table of latent heats[link]

The following table shows the latent heats and change of phase temperatures of some common fluids and gases.

Latent heat for condensation of water[link]

The latent heat of condensation of water in the temperature range from −40 °C to 40 °C is approximated by the following empirical cubic function:

Failed to parse (Missing texvc executable; please see math/README to configure.): L_{water}(T)=-0.0000614342 T^3+0.00158927 T^2-2.36418 T+2500.79

^[9] [J / g] with a determination coefficient of Failed to parse (Missing texvc executable; please see math/README to configure.): R^2=0.999988 , where Failed to parse (Missing texvc executable; please see math/README to configure.): T

is in °C

See also[link]

• Bowen ratio
• Eddy covariance flux (eddy correlation, eddy flux)
• Sublimation (physics)
• Specific heat capacity
• Enthalpy of fusion
• Enthalpy of vaporization

References[link]

v t e States of matter Solid Liquid Gas / Vapor Plasma Low energy Bose–Einstein condensate · Fermionic condensate · Quantum Hall · Rydberg matter · Strange matter · Superfluid · Supersolid High energy Degenerate matter · QCD matter · Quark–gluon plasma  · Supercritical fluid Other states Colloid · Glass · Liquid crystal · Magnetically ordered (Antiferromagnet, Ferrimagnet, Ferromagnet) · String-net liquid · Superglass Transitions Boiling · Boiling point · Condensation · Critical line · Critical point · Crystallization · Deionization · Deposition · Evaporation · Flash evaporation · Freezing · Ionization · Lambda point · Melting · Melting point · Regelation · Saturated fluid · Sublimation · Supercooling · Triple point · Vaporization Quantities Enthalpy of fusion · Enthalpy of sublimation · Enthalpy of vaporization · Latent heat · Latent internal energy · Trouton's ratio · Volatility Concepts Binodal · Compressed fluid · Cooling curve · Equation of state · Leidenfrost effect · Mpemba effect · Order and disorder (physics) · Spinodal · Superconductivity · Superheated vapor · Superheating · Thermo-dielectric effect