What do you understand by term latent heat?

Latent heat is the heat required for an object to change phase (melt, boil, freeze, etc.). This energy is closely related to enthalpy.[1] In figure 1, very cold ice has heat added to it. The temperature goes up, so that's sensible heat, but once it starts melting, that heat is latent heat (and is represented by the flat parts of the line, during melting or evaporation).

What do you understand by term latent heat?

Figure 1. Adding heat to water can either raise the temperature or change the phase.[2] The heat that changes the temperature is sensible heat, the heat that changes the phase is latent heat.

Fusion

The latent heat of fusion is the heat required for an object to go from the solid state to the liquid state, or vice versa.[1] Since its value is generally much higher than specific heat, it allows you to keep a beverage cold for much longer by adding ice than simply having a cold liquid to begin with. It's also why frozen meat takes a long time to thaw, but once its thawed, it heats up quickly. The latent heat of fusion is also referred to as the enthalpy of fusion.

Ice and water have enormous latent heats associated with them which is why snow takes so long to melt and boiling water is used for cooking. This is also important in keeping our planet comfortable to live on, and provides a fair amount of resistance to climate change.

Vaporization

The latent heat of vaporization is the thermal energy required for a liquid to vaporize to a gas or the amount that is released when a gas condenses to a liquid.[1] The latent heat of vaporization is also referred to as the enthalpy of vaporization. Water has a high latent heat of vaporization, which is why steam burns are so dangerous. When steam burns a person's arm for example, this energy transfer causes the steam to condense—which uses much more energy than simply changing the temperature. Hot water would also burn, however, if a person were to get a hot water burn—that would only change the temperature of the person's hand and the water, and will not cause a phase change. Therefore, steam water burns are much worse (because a phase change is actually happening).

Specific Latent Heat

What do you understand by term latent heat?

Figure 2: Specific latent heat of fusion, vaporization and sublimation.[3]

The specific latent heat of a substance is the amount of energy needed to change the state of 1 kg of the substance without changing its temperature.[4] The distinction from latent heat is due to the term "specific," which tells us that the substance is being measured per unit mass. In addition, since specific latent heat is measured per unit mass, it is an intensive property of matter.

The specific latent heat (represented by the symbol [math]L[/math]) of a material is a measure of the heat energy ([math]Q[/math]) per unit mass ([math]m[/math]) released or absorbed during a phase change. Specific latent heat can be found using the formula [math]Q = mL[/math]. Rearranging the formula we get: [math]L = \frac{Q}{m}[/math]

where:

  • [math]L[/math] is the specific latent heat for a particular substance, either Lf for fusion, or Lv for vaporization.
  • [math]Q[/math] is the thermal energy released or absorbed during the phase change of the substance (usually in kJ).
  • [math]m[/math] is the mass of the substance (usually in kg).


To learn more about enthalpy and latent heat please see hyperphysics.

For Further Reading

References

  1. ↑ 1.0 1.1 1.2 Randall Knight, Physics for Scientists and Engineers, 3rd Ed. New York: Pearson, 2013, Ch. 17, p. 482.
  2. Wikimedia Commons [Online], Available: https://commons.wikimedia.org/wiki/File:Water_Phase_Change_Diagram.png#/media/File:Water_Phase_Change_Diagram.png
  3. "Latent heat", Cambridge University Press, 2014. [Online]. Available: https://static.cambridge.org/binary/version/id/urn:cambridge.org:id:binary:20190207075706603-0570:9781107326200:61588figu30.png?pub-status=live. [Accessed: 06-May-2021].
  4. Specific latent heat, BBC Bitesize, 2021. [Online]. Available: https://www.bbc.co.uk/bitesize/guides/zg6bdxs/revision/4. [Accessed: 06-May-2021].

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What do you understand by the term 'latent heat' ? What are the two types of latent heat ?

Latent heat: The heat energy that has to be supplied to change the state of a substance without changing the temperature of the substance is called its latent heat.

The two types of latent heat are:

  1. Latent heat of fusion
  2. Latent heat of vaporisation

Concept: Change of State of Matter

  Is there an error in this question or solution?

Thermodynamic phase transition energy

Latent heat (also known as latent energy or heat of transformation) is energy released or absorbed, by a body or a thermodynamic system, during a constant-temperature process — usually a first-order phase transition.

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 and latent heat of vaporization involved in phase changes, i.e. a substance condensing or vaporizing at a specified temperature and pressure.[1][2]

The term was introduced around 1762 by Scottish chemist Joseph Black. It is derived from the Latin latere (to lie hidden). Black used the term in the context of calorimetry where a heat transfer caused a volume change in a body while its temperature was constant.

In contrast to latent heat, sensible heat is energy transferred as heat, with a resultant temperature change in a body.

Usage

What do you understand by term latent heat?

Graph of temperature of phases of water heated from −100 °C to 200 °C – the dashed line example shows that melting and heating 1 kg of ice at −50 °C to water at 40 °C needs 600 kJ

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, condensation, freezing or melting, 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.[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] Latent heat is energy released or absorbed, by a body or a thermodynamic system, during a constant-temperature process. Two common forms of latent heat are latent heat of fusion (melting) and latent heat of vaporization (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, 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 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.

Meteorology

In meteorology, latent heat flux is the flux of energy 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

The English word latent comes from Latin latēns, meaning lying hidden.[5][6] The term latent heat was introduced into calorimetry around 1750 by Joseph Black—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).[7]

Later, James Prescott Joule 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,[8] relating the latter to thermal energy.

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 1kg, of a substance as an intensive property:

L = Q m . {\displaystyle L={\frac {Q}{m}}.}
What do you understand by term latent heat?

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

Q = m L {\displaystyle Q={m}{L}}
What do you understand by term latent heat?

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−1 or in BTU lb−1), either Lf for fusion, or Lv 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.[citation needed]

Substance SLH of
fusion
(kJ/kg)
Melting
point
(°C)
SLH of
vaporization
(kJ/kg)
Boiling
point
(°C)
Ethyl alcohol 108 −114 855 78.3
Ammonia 332.17 −77.74 1369 −33.34
Carbon dioxide 184 −78 574
Helium     21 −268.93
Hydrogen(2) 58 −259 455 −253
Lead[9] 23.0 327.5 871 1750
Methane 59 −182.6 511 −161.6
Nitrogen 25.7 −210 200 −196
Oxygen 13.9 −219 213 −183
Refrigerant R134a   −101 215.9 −26.6
Refrigerant R152a   −116 326.5 -25
Silicon[10] 1790 1414 12800 3265
Toluene 72.1 −93 351 110.6
Turpentine     293  
Water 334 0 2264.705 100

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:

L water ( T ) ≈ ( 2500.8 − 2.36 T + 0.0016 T 2 − 0.00006 T 3 )   J/g , {\displaystyle L_{\text{water}}(T)\approx \left(2500.8-2.36T+0.0016T^{2}-0.00006T^{3}\right)~{\text{J/g}},}
What do you understand by term latent heat?
[11]

where the temperature T {\displaystyle T} 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:

L ice ( T ) ≈ ( 2834.1 − 0.29 T − 0.004 T 2 )   J/g . {\displaystyle L_{\text{ice}}(T)\approx \left(2834.1-0.29T-0.004T^{2}\right)~{\text{J/g}}.}
What do you understand by term latent heat?
[11]

Variation with temperature (or pressure)

What do you understand by term latent heat?

Temperature-dependency of the heats of vaporization for water, methanol, benzene, and acetone.

As the temperature (or pressure) rises to the critical point, the latent heat of vaporization falls to zero.

See also

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

References

  1. ^ Perrot, Pierre (1998). A to Z of Thermodynamics. Oxford University Press. ISBN 0-19-856552-6.
  2. ^ Clark, John O.E. (2004). The Essential Dictionary of Science. Barnes & Noble Books. ISBN 0-7607-4616-8.
  3. ^ Bryan, G.H. (1907). Thermodynamics. An Introductory Treatise dealing mainly with First Principles and their Direct Applications, B.G. Tuebner, Leipzig, pages 9, 20–22.
  4. ^ Maxwell, J.C. (1872). Theory of Heat, third edition, Longmans, Green, and Co., London, page 73.
  5. ^ Harper, Douglas. "latent". Online Etymology Dictionary.
  6. ^ Lewis, Charlton T. (1890). An Elementary Latin Dictionary. Entry for latens.
  7. ^ James Burke (1979). "Credit Where It's Due". The Day the Universe Changed. Episode 6. Event occurs at 50 (34 minutes). BBC.
  8. ^ J. P. Joule (1884), The Scientific Paper of James Prescott Joule, The Physical Society of London, p. 274, I am inclined to believe that both of these hypotheses will be found to hold good,—that in some instances, particularly in the case of sensible heat, or such as is indicated by the thermometer, heat will be found to consist in the living force of the particles of the bodies in which it is induced; whilst in others, particularly in the case of latent heat, the phenomena are produced by the separation of particle from particle, so as to cause them to attract one another through a greater space., Lecture on Matter, Living Force, and Heat. May 5 and 12, 1847
  9. ^ Yaws, Carl L. (2011). Yaws' Handbook of Properties of the Chemical Elements. Knovel.
  10. ^ Elert, Glenn (2021). "Latent Heat". The Physics Hypertextbook.
  11. ^ a b Polynomial curve fits to Table 2.1. R. R. Rogers; M. K. Yau (1989). A Short Course in Cloud Physics (3rd ed.). Pergamon Press. p. 16. ISBN 0-7506-3215-1.

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