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In thermochemistry
Thermochemistry

In thermodynamics and physical chemistry, thermochemistry is the study of the energy evolved or absorbed in chemical reactions and any physical transformations, such as melting and boiling....
, latent heat is the amount of energy
Energy

In physics, energy is a scalar physical quantity that describes the amount of Work_ that can be performed by a force. Energy is an attribute of objects and systems that is subject to a conservation law....
 in the form of heat
Heat

In physics and thermodynamics, heat is any transfer of energy from one body or thermodynamic system to another due to a difference in temperature....
 released or absorbed by a chemical substance
Chemical substance

A chemical substance is a material with a specific Empirical formula. It is a concept that became firmly established in the late eighteenth century after work by the chemist Joseph Proust on the composition of some pure chemical compounds such as basic copper carbonate....
 during a change of state
State of matter

States of matter are the distinct forms that different phase take on. Historically, the distinction is made based on qualitative differences in bulk properties....
 (i.e. solid, liquid, or gas), or a phase transition
Phase transition

In thermodynamics, a phase transition is the transformation of a thermodynamic system from one phase to another.At phase-transition point, physical properties may undergo abrupt change- for instance, volume of the two phases may be vastly different....
.

The term was introduced around 1750 by Joseph Black
Joseph Black

Joseph Black was a Scottish physician, physicist, and chemist, known for his discoveries of latent heat, specific heat, and carbon dioxide. He was a founder of thermochemistry who developed many pre-thermodynamics concepts, such as heat capacity, and was the mentor for James Watt....
 as derived from the Latin latere, to lie hidden. The term has now been replaced by "enthalpy
Enthalpy

In thermodynamics and chemistry, the enthalpy is a quotient or description of thermodynamic potential of a system, which can be used to calculate the heat transfer during a quasistatic process taking place in a closed system thermodynamic system under constant pressure....
 of transformation".

Two latent heats (or enthalpies) are typically described: latent heat of fusion (melting
Melting

Melting is a process that results in the phase change of a substance from a solid to a liquid. The internal energy of a solid substance is increased to a specific temperature at which it changes to the liquid phase....
), and latent heat of vaporization (boiling
Boiling

Boiling, a type of phase transition, is the rapid vaporization of a liquid, which typically occurs when a liquid is heated to its boiling point, the temperature at which the vapor pressure of the liquid is equal to the pressure exerted on the liquid by the surrounding environmental pressure....
).






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Encyclopedia


In thermochemistry
Thermochemistry

In thermodynamics and physical chemistry, thermochemistry is the study of the energy evolved or absorbed in chemical reactions and any physical transformations, such as melting and boiling....
, latent heat is the amount of energy
Energy

In physics, energy is a scalar physical quantity that describes the amount of Work_ that can be performed by a force. Energy is an attribute of objects and systems that is subject to a conservation law....
 in the form of heat
Heat

In physics and thermodynamics, heat is any transfer of energy from one body or thermodynamic system to another due to a difference in temperature....
 released or absorbed by a chemical substance
Chemical substance

A chemical substance is a material with a specific Empirical formula. It is a concept that became firmly established in the late eighteenth century after work by the chemist Joseph Proust on the composition of some pure chemical compounds such as basic copper carbonate....
 during a change of state
State of matter

States of matter are the distinct forms that different phase take on. Historically, the distinction is made based on qualitative differences in bulk properties....
 (i.e. solid, liquid, or gas), or a phase transition
Phase transition

In thermodynamics, a phase transition is the transformation of a thermodynamic system from one phase to another.At phase-transition point, physical properties may undergo abrupt change- for instance, volume of the two phases may be vastly different....
.

The term was introduced around 1750 by Joseph Black
Joseph Black

Joseph Black was a Scottish physician, physicist, and chemist, known for his discoveries of latent heat, specific heat, and carbon dioxide. He was a founder of thermochemistry who developed many pre-thermodynamics concepts, such as heat capacity, and was the mentor for James Watt....
 as derived from the Latin latere, to lie hidden. The term has now been replaced by "enthalpy
Enthalpy

In thermodynamics and chemistry, the enthalpy is a quotient or description of thermodynamic potential of a system, which can be used to calculate the heat transfer during a quasistatic process taking place in a closed system thermodynamic system under constant pressure....
 of transformation".

Two latent heats (or enthalpies) are typically described: latent heat of fusion (melting
Melting

Melting is a process that results in the phase change of a substance from a solid to a liquid. The internal energy of a solid substance is increased to a specific temperature at which it changes to the liquid phase....
), and latent heat of vaporization (boiling
Boiling

Boiling, a type of phase transition, is the rapid vaporization of a liquid, which typically occurs when a liquid is heated to its boiling point, the temperature at which the vapor pressure of the liquid is equal to the pressure exerted on the liquid by the surrounding environmental pressure....
). The names describe the direction of heat flow from one phase to the next: solid ? liquid ? gas.

The change is endothermic
Endothermic

In thermodynamics, the word endothermic "within-heating" describes a process or reaction that absorbs energy in the form of heat. Its etymology stems from the Greek prefix endo-, meaning ?inside? and the Greek suffix ?thermic, meaning ?to heat?....
, i.e. the system absorbs energy, when the change is from solid to liquid to gas. It is exothermic
Exothermic

File:Explosion1.JPG In thermodynamics, the term exothermic describes a process or reaction that releases energy usually in the form of heat, but also in form of light , electricity , or sound....
 (the process releases energy) when it is in the opposite direction. For example, in the atmosphere
Earth's atmosphere

The Earth's atmosphere is a layer of gases surrounding the planet Earth that is retained by the Earth's gravity. Dry air contains roughly 78.08% nitrogen, 20.95% oxygen, 0.93% argon, 0.038% Carbon dioxide in the Earth's atmosphere, and trace amounts of other gases....
, when a molecule of water
Water

Water is a common chemical substance that is essential for the survival of all known forms of life. In typical usage, water refers only to its liquid form or States of matter, but the substance also has a solid state, ice, and a gaseous state, water vapor or steam....
 evaporates from the surface of any body of water, energy is transported by the water molecule into a lower temperature air parcel
Air parcel

An air parcel is an imaginary volume of air used by meteorologists to conceptualize the thermodynamic fluid motions of the Earth's atmosphere for use in weather forecasting....
 that contains more water vapor
Water vapor

Water vapor or water vapour , also aqueous vapor, is the gas phase of water . Water vapor is one Phase of the water cycle within the hydrosphere....
 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
Phase (matter)

In the physical sciences, a phase is a region of space , throughout which all physical properties of a material are essentially uniform. Examples of physical properties include density, refractive index, and chemical composition....
 onto a surface, the latent energy absorbed during evaporation is released as sensible heat
Sensible heat

Sensible heat is potential energy in the form of thermal energy or heat. The thermal body must have a temperature higher than its surroundings ....
 onto the surface. The large value of the enthalpy of condensation of water vapor is the reason that steam is a far more effective heating medium than boiling water, and is more hazardous.

Latent heat equation


The equation for latent heat is:


where:

Q is the amount of energy released or absorbed during the change of phase of the substance (in joule
Joule

The joule is the SI derived unit of energy in the International System of Units. It is defined as:One joule is the amount of energy required to perform the following actions:...
s),
m is the mass of the substance,
L is the specific latent heat for a particular substance (J kg-1).


In other words, specific latent heat is found when energy is divided by mass.

Table of latent heats


See also

  • Heat of vaporization
  • Heat of fusion
  • Bowen ratio
    Bowen ratio

    A Bowen ratio is the ratio of energy fluxes from one medium to another by sensible heat and latent heat heating respectively. It is calculated by the equation...
  • Sublimation (physics)
    Sublimation (physics)

    Sublimation of an element or compound is a transition from the solid to gas phase with no intermediate liquid stage. Sublimation is an endothermic phase transition that occurs at temperatures and pressures below the triple point ....
  • Specific heat capacity
    Specific heat capacity

    Specific heat capacity, also known simply as specific heat, is the measure of the energy required to increase the temperature of a of a substance by a certain Celsius#Temperatures_and_intervals....