Talk:Latent heat

Error in data from external link

As far as I can tell, (at least some of) the data in the external link "Latent Heat of Melting for some common Materials" (http://www.engineeringtoolbox.com/latent-heat-melting-solids-d_96.html) is quite wrong!
This is underscored by the page having a "Related" link near the bottom labeled "Melting and Boiling Temperatures - Evaporation and Melting Heat" (http://www.engineeringtoolbox.com/melting-boiling-temperatures-d_392.html) which has different values for some of the same entities! This is two pages on the same site!!
It seems to clearly make the source questionable.
I would have mailed the webmaster of engineeringtoolbox, but I can't find a contact link.

Example: The "latent heat of of melting" for Copper is listed as 176 kJ/kg (or 75.6 Btu/lb, haven't checked this). On the related page, it is listed as 207 kJ/kg.
(NB: From http://www.webelements.com/webelements/elements/text/Cu/heat.html I get the enthalpy of fusion for Copper as 13,1kJ/mol. Divided by the molar weight of 63.546 gram/mol this yields 206kJ/kg, so the value on the page linked from this wiki page is probably NOT right (but the one on that page's related link is).

I suggest that the link be removed, and I will do so now (this is my first attempt at editing an Wiki, if you disagree with my edit, feel free to undo my changes)

In other news: This page seems to be almost completely redundant with Heat_of_vaporization and Heat_of_fusion (the first two links in See Also), why not move the few pieces not found there, and change this page into a pure disambiguation? Again, I'm hardly the expert on things Wiki.
BTW: Latent_heat_of_fusion and Latent_heat_of_vaporization redirects to the same pages as the two links above, and would more clearly indicate the relationsship with this page.

   - Richard - 83.91.35.211 22:26, 11 April 2007 (UTC)

Query '04

Am I correct to question the "i.e. Temperature" part of the article?

Dylan Shell 09:22, 12 Jul 2004 (UTC)

From my admittedly limited understanding, I believe you are correct. I'll remove your commentary and the reference to temperature and instead wikify internal energy. Thanks for bringing that up, and welcome to Wikipedia! -- Hadal 09:36, 12 Jul 2004 (UTC)

I am a little confused. According to the article, it says solid to gas is endothermic whereas gas to solid to exothermic. Shouldn't it be the other way round?

Dan

Look at it from the point of view of the solid sublimating or liquid evaporating. That material must absorb energy to change phase from solid->liquid->gas. It absorbs energy from its surroundings (cooling them) and is thus in an endothermic process.

Alex. 05:08, 5 August 2006 (UTC)

Latent heat as a function of pressure/temperature

Does the latent heat of a substance change as the pressure increases? Presumably past the critical point it is 0, does it move steadily towards 0 as we move up the gas-liquid curve, or does it drop off non-linearly? njh 11:18, 6 December 2005 (UTC)

I believe the latent heat of fusion will increase/decrease dependent on the expansion/contraction during the fase change between solid and liquid. Ie. an expansion will do work on the surroundings, and this work will be higher at higher ambient pressure.

Most materials contract during freezing - but water is an example of the opposite - it expands during freezing, which is why ice floats on water (expanding means lowering the density). This is a very lucky property of water - otherwise the ocean ice would be at the bottom of the ocean instead of floating - which I hear would prohibit life as we know it (not sure why).

As far as I know vaporization always leads to expansion, so the latent heat of vaporization should at first look always increase with temperature. This however is not the case (as you say, at the critical point latent heat of vaporization is zero), because the expansion will be smaller faced with a higher pressure.

The two effects works in opposite directions, so the net effect will not be linear. At zero pressure the work will be zero because there is no force, and at the critical pressure the work will be zero again because there is no expansion. In between the expansion will do positive work during vaporization. For freezing/melting the relationship should be almost linear as neither the volume of solids nor liquids are significantly affected by pressure (NB: This is by logic only, I didn't go look it up, so I might be wrong) - Richard - 83.91.35.211 22:26, 11 April 2007 (UTC)

I've added a diagram showing how LH falls (non-linearly) to zero at the CP. - Rod57 (talk) 13:23, 29 April 2017 (UTC)

Edit to "Latent heat of water" section. The fit applies only to the temperature range from -40 C to 40 C. The error at 100 C is huge. (Latent heat of vaporization goes to zero at the boiling point...). Also, the footnote has been changed to reflect that whoever put this in actually used a cubic fit, and not a quartic. (My fitting program gives slightly different coefficients and a better fit to the original data, but making a small improvement to an approximation seems like a waste of time...)

 -  G. Nunes  —Preceding unsigned comment added by Gnunesjr (talk • contribs) 16:47, 9 November 2009 (UTC) 

Units need to be listed for this approximation as well, as right now it is ambiguous. 134.84.19.236 (talk) 18:40, 10 June 2010 (UTC)

Latent heat and global warming

Has anyone calculated the amount of latent heat required to melt the icecaps? And what about the time it may take to absorb all that latent heat.. How far has the process gone? Gregorydavid 15:57, 1 May 2006 (UTC)

About 30e6 km^3 of ice, so about 1e23 joules. Total solar radiation is ~1e17 W, so...about a million seconds (if, if, if, etc.). At any rate, don't expect the latent heat of the polar caps to hold anything back.... (Hmmm. This is getting forum-y. I'll stop now.) Gnunesjr (talk) 14:39, 10 November 2009 (UTC)

Section line

the line under one of the sections cuts into the template on the right (at least in firefox). Is there a clean way to make the line shorter? Ojcit 21:08, 2 October 2006 (UTC)

Latent heat of vaporization for water

http://en.wikipedia.org/wiki/Kelvin_temperature_scale

Water’s boiling point 373.1339 K

http://webbook.nist.gov/cgi/fluid.cgi?TLow=363.1339&THigh=383.1339&TInc=10&ID=C7732185&Action=Load&Type=SatP&TUnit=K&PUnit=MPa&DUnit=mol%2Fl&HUnit=kJ%2Fkg&WUnit=m%2Fs&VisUnit=uPa%2As&STUnit=N%2Fm&RefState=DEF

Enthalpy (kJ/kg):

• Vapor = 2675.5
• Liquid = 419.10

difference: 2675.5 - 419.1 = 2256.4 kJ/kg

-Ac44ck (talk) 06:50, 24 February 2008 (UTC)

Units

No unit is given in the section "Latent heat for water". I believe it is kJ/kg, but maybe someone could check and add this information. --139.75.1.45 (talk) 08:11, 8 August 2011 (UTC)

Latin origin

I apparently need to explain this because the change was reverted and someone even had the nerve to call it vandalism. Before my edits, the article said:

The term latent heat is derived from the Latin latere, meaning to lie hidden.

This is not correct. The term latent is derived from Latin as stated above, however the word latent was first used around the 15th century. Thus the meaning of the word had been established for over one hundred years before Black applied it to thermodynamics, creating the term "latent heat". Thus, the term "latent heat" did not derive from the Latin because the meaning of the word latent was already pre-established. To reiterate my main point, Black did not invent the word latent and as such, did not derive it from anything.

--Transce080 (talk) 18:07, 14 September 2011 (UTC)

Typo?

Quoted from this section: Specific latent heat "Intensive properties are material characteristics and are not dependent on the size or extend of the sample". Does "extend" make sense or is it supposed to be "extent"? — Preceding unsigned comment added by 197.237.15.233 (talk) 13:22, 7 January 2012 (UTC)

Conflict and redundancies

¿Why are there separate articles for Enthalpy of fusion (and Enthalpy of vaporization) and Latent heat, when they describe essentially the same concept? To make matters worse, the data in the tables are different (108 kJ/kg for ethanol in the article on "Latent Heat" and 109 kJ/kg in the article about enthalpy). --Gonfer00 (talk) 09:10, 25 February 2012 (UTC)

I have moved this new item to the usual place for such items, at the bottom of the page, from the top where it was originally placed by an editor apparently not familiar with the usual arrangement for such items.Chjoaygame (talk) 09:31, 25 February 2012 (UTC)

It is an opinion of editor Gonfer that enthalpy of fusion, enthalpy of vaporization and latent heat "describe essentially the same concept". Some thought may change his mind. The concept of latent heat is of historical origin, from a time long before even the establishment of the law of conservation of energy, when interconvertibility of heat and work was not understood. In contrast, enthalpy is a concept of thermodynamics, based on the knowledge of the interconvertibility of heat and work for thermodynamic systems, a conceptual frame that arose much later than the concept of latent heat. The difference between vaporization and fusion is clear. It is a general principle that a thing is what it is and not something else. The reality and validity of a conceptual linkage between the three items that editor Gonfer refers to does not make them the same. If it really did, editor Gonfer would not feel the need to put in the word "essentially" which serves to show that it is his opinion that the three concepts have a common "essence".Chjoaygame (talk) 09:47, 25 February 2012 (UTC)

Practicle uses

I note that there is no mention of the practicle or natural uses of Latent Heat.

Heat transfer using Latent Heat is now so widespread with most homes having fridges and freezers, cars having a/c units and more and more industrial applications being developed all the time.

In a simple heat engine, the Evaporator will absorb heat energy as the refrigerant inside boils from liquid to vapour, and on the other side of the system the Condenser will reject heat as the vapour inside condenses to a liquid. Both states in the Evaporator and the Condensor are when the refrigerant property is at its Latent Heat stage, in all other parts of the system the refrigerant property is either a liquid (Sub cooled) or vapour (Super Heated). The heat engine works by changing the pressure of the refrigerant at different stages.

Sweating or perspiration so that the sweat evaporates and cools the body.

Medical burn treatment packs where the cooling process is via evaporation.

The list is not exhaustive.

I believe that the simple properties that occur when a substance goes through the Latent Heat stage particularly whilst boiling hold the key to cheaper energy solutions. — Preceding unsigned comment added by 86.167.81.85 (talk) 22:32, 7 March 2012 (UTC)

Questionable formula for latent heat of water vaporization

There is a nice cubic polynomial supposedly valid from -40C to +40C. It does not distinguish between ice and liquid water. I have to assume that it applies to a supercooled liquid below freezing point. A word of caution should be added to the formula.

Consider 1 kg of water ice at 0 degrees. To vaporize it, you can melt it to 1 kg of liquid water at 0 degrees - you have to supply a latent heat of melting. Then you can vaporize it from a liquid phase. There should be a jump in the latent heat of vaporization (LHV)- at 0 degrees, LHV for ice should be higher by a latent heat of melting than LHV for liquid water. The polynomial does not exhibit this behavior. It may be correct for temperatures above freezing.

67.169.96.95 (talk) 16:13, 4 June 2012 (UTC)

two verbs

There are two verbs, to effect, and to affect, recognizably distinct. The one that fits here is to effect. Partly cribbing from the Oxford English Dictionary, not reproducing all that it says, the distinction is as follows. One effects a process by putting it into effect. One affects a process by changing its character. Thus, for example, one can affect a process by slowing it down, but this is not effecting it. Effecting a process is setting it going and seeing it to completion. The latter is relevant in the article where it speaks of "completely effecting a phase change". It hardly makes sense to speak of 'completely affecting a phase change', because it hardly makes sense to think of changing the character of a phase change.Chjoaygame (talk) 03:48, 17 January 2013 (UTC)

Article is a bit muddled up on the practical meaning of latent heat

Latent heat of fusion confusingly means latent heat of melting

Latent heat cannot be 'released'. It can only be 'extracted' by providing a colder environment for the phase change energy to flow to.

For example if wet air rises it warms the colder air that is not rising - the rising air does not become warmer. Similarly in a thunderstorm the storm is not releasing heat in the sense it gets hotter. Instead the rising core of the storm is able to rise more strongly than the outer core because:

1. the core has not cooled as rapidly as the air outside the storm because the latent heat contained in the water vapour has kept the air warmer than otherwise

2. The storm contains humid air which is less dense than drier air

3. The blanket of water must act to insulate the warmer air mass inside the core of the storm from an ability to radiate energy to space.

Steam is more burning than water at the same temperature because your skin needs to use more work to cool steam than it needs to use to cool water of the same temperature. You dont get burnt by the steam releasing a heating power that the water does not possess. Instead you just get burnt for longer at the same temperature.

In meterology the different dryness levels of the atmosphere create confusion when talking about latent heat. Water will cool if it evaporates into a very dry atmosphere, your skin is only cooler when you sweat if the air is drier further away from your body. At a certain humidity level there is only an equilibrium exchange of gas and liquid.

Latent heat transports energy with the water into the air but condensation cannot release a heating energy other than an ability to slow down the rate of cooling when the water vapour cools.

As an aside, I note that so called experts on the Arctic are talking about release of latent heat of fusion when water freezes where drawings are produced showing a latent heat flux wafting up into the atmosphere! — Preceding unsigned comment added by 88.114.205.194 (talk) 08:36, 17 September 2013 (UTC)

request "don't remove" made by IP editor for faulty edit

The IP edit shown here is faulty and should be undone. It is a second IP posting of the same edit, which I just previously undid. The repeat IP posting failed to say anything on this talk page. I wish to prevent repeated undo problems.

The edit is logically faulty because it mentions latent heat only in the phrase "in latent heat", which has no meaning in the context.

The edit should be undone.Chjoaygame (talk) 03:27, 17 August 2015 (UTC)

Removed. Vsmith (talk) 14:20, 17 August 2015 (UTC)

Could the Specific latent heat for condensation of water section mention pressure

Specific latent heat for condensation of water gives functions of temperature. It seems to just use references for cloud physics - so I've qualified the section title since it is not clear if the formulae apply to all pressures or just those typically found in clouds/cloud-formation. Can we have an explanation of why the formulae do not mention pressure - or say if the formulae only apply at the pressures when visible droplets or ice crystals form ? Perhaps this should be a wider LH in meteorology section ? - Rod57 (talk) 13:16, 29 April 2017 (UTC)

How is latent heat of vaporization so high?

The latent heat of vaporization of water, when measured with calorimetry, turns out to be about 2.25 MJ/kg. Now at 20 °C and 1 atmosphere, water vapor molecules travel at some 590 m/s RMS or 1300 mph, so one could expect their kinetic energy to be converted to sensible heat when brought abruptly to a halt. However that only accounts for about 0.175 MJ/kg (just solve KE = ½mv² where m = 1 kg and KE = 2.25 MJ). Where does the other 92% of 2.25 MJ/kg come from? Can this be computed from the known physics of water molecules? If that physics is known via some WP:RS, wouldn't that be a suitable addition to this article? And if it is not known, wouldn't that be a nice research problem for some physics master's student? Vaughan Pratt (talk) 03:56, 6 April 2023 (UTC)