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Latent heat
In science, latent heat is a heat flow to a body that results in no change in temperature in the body, with specific respect to heat flows that accompany phase transitions such boiling or freezing. [1] In technical terms, latent heat is the heat quantity absorbed or released by a substance during a change of state. [2] Modern terminology, however, for phase transitions that occur at constant temperature and pressure, tends to use the term “enthalpy of transformation” instead of the more obsolete term “latent heat”.
History
The term and phenomenon of latent heat was coined in the chemistry lectures, beginning in 1757, of Scottish physicist and chemist Joseph Black to account for inconsistencies in the then theories on how heating and melting occurred in relation to heat movements and temperature recordings. [3] The older established view of heat flow in relation to phase change, according to Black, when he began to lecture, was that:
The newer view of heat in relation to a solid to liquid phase change, according Black's recent experiments, is that:
Conversely, the newer view of heat in relation to a liquid to solid phase change, according to Black, is that:
To sum his view up, he introduces a new term:
1865 views
Into the 1860s, German physicist Rudolf Clausius expressed his views on heat and latent heat, to the effect “the heat actually present in a unit weight of [a] substance [is] the vis viva of it molecular motions” and that latent heat “is not only, as its name imports, hidden from our perceptions, but has actually no physical existence … it has been converted into work.” [4]
References
1. Giunta, Carmen. (2003). “Black heat capacity”, Le Moyne College, Department of Chemistry.
2. Perrot, Pierre. (1998). A to Z of Thermodynamics, Oxford: Oxford University Press.
3. Black, Joseph. (1803). “Lectures on the Elements of Chemistry” (excerpts on “latent heat” and “specific heat”), University of Edinburgh, published from his manuscripts by John Robison (1803) [as excerpted by William Francis Magie, A Source Book in Physics (New York: McGraw-Hill, 1935)].
4. Maxwell, James C. (1878). “Tait’s ‘Thermodynamics’ (I)”, (pgs. 257-59). Nature, Jan. 31.
History
The term and phenomenon of latent heat was coined in the chemistry lectures, beginning in 1757, of Scottish physicist and chemist Joseph Black to account for inconsistencies in the then theories on how heating and melting occurred in relation to heat movements and temperature recordings. [3] The older established view of heat flow in relation to phase change, according to Black, when he began to lecture, was that:
"Fluidity was universally considered as produced by a small addition to the quantity of heat which a body contains, when it is once heated up to its melting point; and the return of such a body to a solid state, as depending on a very small diminution of the quantity of heat, after it is cooled to the same degree; that a solid body, when it is changed into a fluid, receives no greater addition to the heat within it than what is measured by the elevation of temperature indicated after fusion by the thermometer; and that, when the melted body is again made to congeal, by a diminution of its heat, it suffers no greater loss of heat than what is indicated also by the simple application to it of the same instrument."
The newer view of heat in relation to a solid to liquid phase change, according Black's recent experiments, is that:
"The opinion I formed from attentive observation of the facts and phenomena, is as follows. When ice, for example, or any other solid substance, is changing into a fluid by heat, I am of opinion that it receives a much greater quantity of heat than what is perceptible in it immediately after by the thermometer. A great quantity of heat enters into it, on this occasion, without making it apparently warmer, when tried by that instrument. This heat, however, must be thrown into it, in order to give it the form of a fluid; and I affirm, that this great addition of heat is the principal, and most immediate cause of the fluidity induced."
Conversely, the newer view of heat in relation to a liquid to solid phase change, according to Black, is that:
"On the other hand, when we deprive such a body of its fluidity again, by a diminution of its heat, a very great quantity of heat comes out of it, while it is assuming a solid form, the loss of which heat is not to be perceived by the common manner of using the thermometer. The apparent heat of the body, as measured by that instrument, is not diminished, or not in proportion to the loss of heat which the body actually gives out on this occasion; and it appears from a number of facts, that the state of solidity cannot be induced without the abstraction of this great quantity of heat. And this confirms the opinion, that this quantity of heat, absorbed, and, as it were, concealed in the composition of fluids, is the most necessary and immediate cause of their fluidity."
To sum his view up, he introduces a new term:
“[In the] common process of freezing water, the extrication and emergence of the latent heat, if I may be allowed to use these terms, is performed by such minute steps, or rather with such a smooth progress, that many may find difficulty in apprehending it; but [other] example[s] [occur], in which this extrication of the concealed heat becomes manifest and striking.”
1865 views
Into the 1860s, German physicist Rudolf Clausius expressed his views on heat and latent heat, to the effect “the heat actually present in a unit weight of [a] substance [is] the vis viva of it molecular motions” and that latent heat “is not only, as its name imports, hidden from our perceptions, but has actually no physical existence … it has been converted into work.” [4]
References
1. Giunta, Carmen. (2003). “Black heat capacity”, Le Moyne College, Department of Chemistry.
2. Perrot, Pierre. (1998). A to Z of Thermodynamics, Oxford: Oxford University Press.
3. Black, Joseph. (1803). “Lectures on the Elements of Chemistry” (excerpts on “latent heat” and “specific heat”), University of Edinburgh, published from his manuscripts by John Robison (1803) [as excerpted by William Francis Magie, A Source Book in Physics (New York: McGraw-Hill, 1935)].
4. Maxwell, James C. (1878). “Tait’s ‘Thermodynamics’ (I)”, (pgs. 257-59). Nature, Jan. 31.
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