Latent heat (also known as latent energy or heat of transformation) is energy released or absorbed, by a body or a
thermodynamic system
A thermodynamic system is a body of matter and/or radiation, confined in space by walls, with defined permeabilities, which separate it from its surroundings. The surroundings may include other thermodynamic systems, or physical systems that are ...
, during a constant-temperature process — usually a
first-order phase transition
In chemistry, thermodynamics, and other related fields, a phase transition (or phase change) is the physical process of transition between one state of a medium and another. Commonly the term is used to refer to changes among the basic states o ...
.
Latent heat can be understood as energy in hidden form which is supplied or extracted to change the state of a substance without changing its temperature. Examples are
latent heat of fusion
In thermodynamics, the enthalpy of fusion of a substance, also known as (latent) heat of fusion, is the change in its enthalpy resulting from providing energy, typically heat, to a specific quantity of the substance to change its state from a s ...
and
latent heat of vaporization
The enthalpy of vaporization (symbol ), also known as the (latent) heat of vaporization or heat of evaporation, is the amount of energy (enthalpy) that must be added to a liquid substance to transform a quantity of that substance into a gas. T ...
involved in
phase changes
In chemistry, thermodynamics, and other related fields, a phase transition (or phase change) is the physical process of transition between one state of a medium and another. Commonly the term is used to refer to changes among the basic states of ...
, i.e. a substance condensing or vaporizing at a specified temperature and pressure.
The term was introduced around 1762 by
Scottish chemist
A chemist (from Greek ''chēm(ía)'' alchemy; replacing ''chymist'' from Medieval Latin ''alchemist'') is a scientist trained in the study of chemistry. Chemists study the composition of matter and its properties. Chemists carefully describe t ...
Joseph Black
Joseph Black (16 April 1728 – 6 December 1799) was a Scottish physicist and chemist, known for his discoveries of magnesium, latent heat, specific heat, and carbon dioxide. He was Professor of Anatomy and Chemistry at the University of Glas ...
. It is derived from the Latin ''latere'' (''to lie hidden''). Black used the term in the context of
calorimetry
In chemistry and thermodynamics, calorimetry () is the science or act of measuring changes in ''state variables'' of a body for the purpose of deriving the heat transfer associated with changes of its state due, for example, to chemical reac ...
where a heat transfer caused a volume change in a body while its temperature was constant.
In contrast to latent heat,
sensible heat
Sensible heat is heat exchanged by a body or thermodynamic system in which the exchange of heat changes the temperature of the body or system, and some macroscopic variables of the body or system, but leaves unchanged certain other macroscopic vari ...
is energy transferred as
heat
In thermodynamics, heat is defined as the form of energy crossing the boundary of a thermodynamic system by virtue of a temperature difference across the boundary. A thermodynamic system does not ''contain'' heat. Nevertheless, the term is ...
, with a resultant temperature change in a body.
Usage
The terms ″sensible heat″ and ″latent heat″ refer to energy transferred between a body and its surroundings, defined by the occurrence or non-occurrence of temperature change; they depend on the properties of the body. ″Sensible heat″ is ″sensed″ or felt in a process as a change in the body's temperature. ″Latent heat″ is energy transferred in a process without change of the body's temperature, for example, in a phase change (solid/liquid/gas).
Both sensible and latent heats are observed in many processes of transfer of energy in nature. Latent heat is associated with the change of phase of atmospheric or ocean water,
vaporization
Vaporization (or vaporisation) of an element or compound is a phase transition from the liquid phase to vapor. There are two types of vaporization: evaporation and boiling. Evaporation is a surface phenomenon, whereas boiling is a bulk phenomenon ...
,
condensation,
freezing
Freezing is a phase transition where a liquid turns into a solid when its temperature is lowered below its freezing point. In accordance with the internationally established definition, freezing means the solidification phase change of a liquid ...
or
melting
Melting, or fusion, is a physical process that results in the phase transition of a substance from a solid to a liquid. This occurs when the internal energy of the solid increases, typically by the application of heat or pressure, which inc ...
, whereas sensible heat is energy transferred that is evident in change of the temperature of the atmosphere or ocean, or ice, without those phase changes, though it is associated with changes of pressure and volume.
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.
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''.
''Latent heat'' is energy released or absorbed, by a body or a
thermodynamic system
A thermodynamic system is a body of matter and/or radiation, confined in space by walls, with defined permeabilities, which separate it from its surroundings. The surroundings may include other thermodynamic systems, or physical systems that are ...
, during a constant-temperature process.
Two common forms of latent heat are
latent heat of fusion
In thermodynamics, the enthalpy of fusion of a substance, also known as (latent) heat of fusion, is the change in its enthalpy resulting from providing energy, typically heat, to a specific quantity of the substance to change its state from a s ...
(
melting
Melting, or fusion, is a physical process that results in the phase transition of a substance from a solid to a liquid. This occurs when the internal energy of the solid increases, typically by the application of heat or pressure, which inc ...
) and
latent heat of vaporization
The enthalpy of vaporization (symbol ), also known as the (latent) heat of vaporization or heat of evaporation, is the amount of energy (enthalpy) that must be added to a liquid substance to transform a quantity of that substance into a gas. T ...
(
boiling). These names describe the direction of energy flow when changing from one phase to the next: from solid to liquid, and liquid to gas.
In both cases the change is
endothermic
In thermochemistry, an endothermic process () is any thermodynamic process with an increase in the enthalpy (or internal energy ) of the system.Oxtoby, D. W; Gillis, H.P., Butler, L. J. (2015).''Principle of Modern Chemistry'', Brooks Cole. ...
, meaning that the system absorbs energy.
For example, when water evaporates, energy is required for the water molecules to overcome the forces of attraction between them, the transition from water to vapor requires an input of energy.
If the vapor then condenses to a liquid on a surface, then the vapor's latent energy absorbed during evaporation is released as the liquid's
sensible heat
Sensible heat is heat exchanged by a body or thermodynamic system in which the exchange of heat changes the temperature of the body or system, and some macroscopic variables of the body or system, but leaves unchanged certain other macroscopic vari ...
onto the surface.
The large value of the
enthalpy
Enthalpy , a property of a thermodynamic system, is the sum of the system's internal energy and the product of its pressure and volume. It is a state function used in many measurements in chemical, biological, and physical systems at a constant ...
of condensation of water vapor is the reason that steam is a far more effective heating medium than boiling water, and is more hazardous.
Meteorology
In
meteorology
Meteorology is a branch of the atmospheric sciences (which include atmospheric chemistry and physics) with a major focus on weather forecasting. The study of meteorology dates back millennia, though significant progress in meteorology did no ...
, latent heat flux is the
flux of energy from the Earth's surface to the
atmosphere that is associated with
evaporation or
transpiration
Transpiration is the process of water movement through a plant and its evaporation from aerial parts, such as leaves, stems and flowers. Water is necessary for plants but only a small amount of water taken up by the roots is used for growth ...
of water at the surface and subsequent
condensation of
water vapor
(99.9839 °C)
, -
, Boiling point
,
, -
, specific gas constant
, 461.5 J/( kg·K)
, -
, Heat of vaporization
, 2.27 MJ/kg
, -
, Heat capacity
, 1.864 kJ/(kg·K)
Water vapor, water vapour or aqueous vapor is the gaseous p ...
in the
troposphere
The troposphere is the first and lowest layer of the atmosphere of the Earth, and contains 75% of the total mass of the planetary atmosphere, 99% of the total mass of water vapour and aerosols, and is where most weather phenomena occur. From ...
. It is an important component of Earth's surface energy budget. Latent heat flux has been commonly measured with the
Bowen ratio The Bowen ratio is used to describe the type of heat transfer for a surface that has moisture. Heat transfer can either occur as sensible heat (differences in temperature without evapotranspiration) or latent heat (the energy required during a chan ...
technique, or more recently since the mid-1900s by the
eddy covariance
The eddy covariance (also known as eddy correlation and eddy flux) is a key atmospheric measurement technique to measure and calculate vertical turbulent fluxes within atmospheric boundary layers. The method analyses high-frequency wind and scal ...
method.
History
The English word ''
latent
Latency or latent may refer to:
Science and technology
* Latent heat, energy released or absorbed, by a body or a thermodynamic system, during a constant-temperature process
* Latent variable, a variable that is not directly observed but inferred ...
'' comes from Latin ''
latēns'', meaning ''lying hidden''. The term ''latent heat'' was introduced into calorimetry around 1750 by
Joseph Black
Joseph Black (16 April 1728 – 6 December 1799) was a Scottish physicist and chemist, known for his discoveries of magnesium, latent heat, specific heat, and carbon dioxide. He was Professor of Anatomy and Chemistry at the University of Glas ...
—commissioned by producers of
Scotch whisky in search of ideal quantities of fuel and water for their distilling process—to studying system changes, such as of volume and pressure, when the thermodynamic system was held at constant temperature in a thermal bath. Black would compare the change in temperature of two identical quantities of water, heated by identical means, one of which was, say, melted from ice, whereas the other was heated from merely cold liquid state. By comparing the resulting temperatures, he could conclude that, for instance, the temperature of the sample melted from ice was 140°F lower than the other sample, thus melting the ice absorbed 140 "degrees of heat" that could not be measured by the thermometer, yet needed to be supplied, thus it was "latent" (hidden). Black also deduced that as much latent heat as was supplied into boiling the distillate (thus giving the quantity of fuel needed) also had to be absorbed to condense it again (thus giving the cooling water required).
Later,
James Prescott Joule
James Prescott Joule (; 24 December 1818 11 October 1889) was an English physicist, mathematician and brewer, born in Salford, Lancashire. Joule studied the nature of heat, and discovered its relationship to mechanical work (see energy). ...
characterised 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,
[, Lecture on Matter, Living Force, and Heat. May 5 and 12, 1847] relating the latter to
thermal energy
The term "thermal energy" is used loosely in various contexts in physics and engineering. It can refer to several different well-defined physical concepts. These include the internal energy or enthalpy of a body of matter and radiation; heat, de ...
.
Specific latent heat
A ''specific'' latent heat (''L'') expresses the amount of energy in the form of heat (''Q'') required to completely effect a phase change of a unit of mass (''m''), usually , of a substance as an
intensive property
Physical properties of materials and systems can often be categorized as being either intensive or extensive, according to how the property changes when the size (or extent) of the system changes. According to IUPAC, an intensive quantity is one ...
:
:
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
:
where:
:''Q'' is the amount of energy released or absorbed during the change of phase of the substance (in
kJ or in
BTU
The British thermal unit (BTU or Btu) is a unit of heat; it is defined as the amount of heat required to raise the temperature of one pound of water by one degree Fahrenheit. It is also part of the United States customary units. The modern SI u ...
),
:''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
−1 or in BTU lb
−1), either ''L''
f for fusion, or ''L''
v for vaporization.
Table of specific latent heats
The following table shows the specific latent heats and change of phase temperatures (at standard pressure) of some common fluids and gases.
Specific latent heat for condensation of water in clouds
The specific latent heat of condensation of water in the temperature range from −25 °C to 40 °C is approximated by the following empirical cubic function:
:
[ Polynomial curve fits to Table 2.1. ]
where the temperature
is taken to be the numerical value in °C.
For
sublimation and
deposition from and into ice, the specific latent heat is almost constant in the temperature range from −40 °C to 0 °C and can be approximated by the following empirical quadratic function:
:
Variation with temperature (or pressure)
As the temperature (or pressure) rises to the
critical point, the latent heat of vaporization falls to zero.
See also
*
Bowen ratio The Bowen ratio is used to describe the type of heat transfer for a surface that has moisture. Heat transfer can either occur as sensible heat (differences in temperature without evapotranspiration) or latent heat (the energy required during a chan ...
*
Eddy covariance
The eddy covariance (also known as eddy correlation and eddy flux) is a key atmospheric measurement technique to measure and calculate vertical turbulent fluxes within atmospheric boundary layers. The method analyses high-frequency wind and scal ...
flux (eddy correlation, eddy flux)
*
Sublimation (physics)
*
Specific heat capacity
In thermodynamics, the specific heat capacity (symbol ) of a substance is the heat capacity of a sample of the substance divided by the mass of the sample, also sometimes referred to as massic heat capacity. Informally, it is the amount of heat t ...
*
Enthalpy of fusion
In thermodynamics, the enthalpy of fusion of a substance, also known as (latent) heat of fusion, is the change in its enthalpy resulting from providing energy, typically heat, to a specific quantity of the substance to change its state from a s ...
*
Enthalpy of vaporization
The enthalpy of vaporization (symbol ), also known as the (latent) heat of vaporization or heat of evaporation, is the amount of energy (enthalpy) that must be added to a liquid substance to transform a quantity of that substance into a gas. T ...
References
{{Authority control
Thermochemistry
Atmospheric thermodynamics
Thermodynamics
Physical phenomena