Talk:Heat capacity

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to do[edit]

Moving the merged article back from Specific heat capacity, as this is the more fundamental quantity. The bulk of the article I've now copied across, and adapted as required to make heat capacity in general the primary focus, rather than specific heat capacity.

Still to do:

  • Correct table: Ammonia is a gas at 25 deg.C and in the table it stands liquid, so either it is typo or measurement temperature need to be provided (should be <33.34 deg.C).
  • Offer a good one paragraph introductory explanation with easily understood examples in layman's terms for a high school to some college educated audience. — Preceding unsigned comment added by 74.215.242.83 (talk) 07:24, 7 December 2014 (UTC)[reply]
  • Background. More qualitative discussion would be good, before jumping into all the scary algebra of the thermodynamics section. fr.wiki has a nice couple of lines on the replacement of the caloric view with the energetic view, which would fit well with a references to history and early works. Done. There is also quite a lot of material in the lead and first section of the specific heat article, which maybe should be adapted and carried over; though it's written very much around specific heat, so I have left it out initially. Now partly done, under "Metrology" and "Alternate units". A little more to do.
  • Earlier main discussion of specific heat capacity, molar heat capacity. It doesn't really fit where it is at the moment, the article would benefit by treating the intensive variants earlier; the "definitions and formal properties" section would also benefit from the streamlining. de.wiki's article is very terse and effective on this, might be worth adapting. Done.
  • Alternative units. Again, the current specific heat article makes quite a lot of this, and they probably should get mentioned. Done. Also the 25°C standard temperature. Probably best just after the discussion of specific heat capacity and molar heat capacity. Done.
  • Lead. Should summarise the whole article. Holding text for the moment; definitely needs revision, but probably easier after the 3 sections above are more in place.
  • Theory of heat capacity. Needs some spring cleaning and streamlining, to make sure the ideas actually flow -- it starts with rather a jump into the middle at the moment.
  • Negative heat capacity. de-wiki has a paragraph on this. Done.
  • Ratio of specific heats. We ought to have a mention of γ.
  • Dimensionless heat capacity. Relate to dimensionless entropy. Review existing "mutual information" reference -- factors look wrong to me. Done

No doubt there's more, but that's enough to start with. Jheald (talk) 11:24, 29 April 2010 (UTC). Updated. Jheald (talk) 13:56, 29 April 2010 (UTC)[reply]

I've added back in a paragraph from the specific heat intro which summarizes the microscopic theory section somewhat. LEDE is too short, as it is, and this won't hurt.

Regarding gamma, it's interested that even the main article Relations between heat capacities doesn't give the ratio of Cp/Cv a name. Or if so, I missed it. Here's the natural place to add it, in both articles. SBHarris 22:09, 29 April 2010 (UTC)[reply]

  • COMPARE RESULTS FROM EXPERIMENT WITH THEORETICAL VALUE —Preceding unsigned comment added by 210.48.147.2 (talk) 03:27, 28 September 2010 (UTC)[reply]
  • Discussion of other intensive heat capacity quantities. The word "specific" means "divided by something". Thus we have mole-specific heat capacity (C/mole), mass-specific heat capacity (C/mass), and (for solids) volume-specific heat capacity (C/volume). The lead of this article blithley assumes that specific heat always means mass-specific heat, when that is not the case at all. I've really had a problem with the other editors on this article. I quote from my engineering heat transfer books, and they simply removed the references and go back to their pet beliefs about what they learned about what things are called. Maybe in their chem and physics classes or texts-- I don't know. But the concept of "specific heat" is used in many fields. And it would be nice if some of people who toss about the word "specific" in the physical sciences, had some idea of THAT term comes from. SBHarris 02:02, 22 November 2010 (UTC)[reply]
This characterization is simply wrong, pov, and not supported by any references. The lone prefix specific is always understood in physics, chemistry, and yes, engineering, as a mass-specific parameter, when no other explanations are explicitly present. IUPAC and the BIPM/SI brochure are also rater clear and unambiguous about this usage, which is now referenced in this article. Mole specific heat capacity is never used, and mass-specific extremely rarely, when it is absolutely needed to distinguish from the volumetric property. Kbrose (talk) 05:48, 22 November 2010 (UTC)[reply]

You could find the references yourself if you bothered. Enter "mole specific heat capacity" into Google scholar and you'll find 16 references to papers and science texts that use the term (is 16 = "never" in your personal number system?) Do the same with "molar specific heat capcity" (use the quotes) and you will get 146 references. Even farther from never, if my memory of the number line serves.

One of your references says that "specific" USUALLY refers to "mass specific," not that it never does or is "always understood" to refer to mass-- you even quoted the usually in your cite; did you understand it? IUPAC now recommends that the lone term "specific heat capacity" always refer to mass-specific heat capacity and that's fine with me. I left that in. But what you say about "The lone prefix specific is always understood in physics, chemistry, and yes, engineering, as a mass-specific parameter, when no other explanations are explicitly present" is contradicted by about 20 perfectly well defined and commonly used engineering and science terms, which have no better explanations than the ones given for intensive specific heat quantities discussed in this article:

Actually there are more than 20 there, but does the number 20 = "never" in your system of term-counting? That's an interesting POV, if true. You know, you could actually learn something about the history and actual terminology in the present day in the sciences, including use of the word "specific," if you were willing to be educated. Any of the terms used above can be "Google Scholar"ed for references to texts and peer-reviewed articles. Do you want to try one, and see? So do it and learn something. Don't just tell me I don't know what I'm talking about. The person who is clueless about terminology here, would be you. SBHarris 07:11, 22 November 2010 (UTC) How long will edits exist in Limbo? I see suggestions and corrections from 2010-date and can't tell whether to suggest fixes that will never be acted upon or to not bother. William Hoffman !100.35.21.51 (talk) 02:42, 23 March 2015 (UTC)[reply]

Sulphur hexafluoride as a building material?[edit]

In the building materials table, gaseous sulphur hexafluoride is listed. Is there a motivation for this or is it vandalism? — Preceding unsigned comment added by 130.232.194.121 (talk) 08:59, 27 February 2015 (UTC)[reply]

It seems to be used in switchboxes as an insulator, but is not necessarily related to the other materials. As such, I have removed it. If other people oppose this, we can reinstate it. The entries were: Sulphur Hexafluoride, gas, 0.664Mgibby5 (talk) 20:44, 11 July 2015 (UTC)[reply]

As far as I know, it's only used to fill double-glazed windows. No other applications in building besides that. --AndreCharles (talk) 03:21, 3 November 2016 (UTC)[reply]

"High Temperature" is vague[edit]

  • "Nevertheless, for sufficiently high temperatures, these degrees of freedom cannot be ignored"
  • "Thus, it is the heat capacity per-mole-of-atoms, not per-mole-of-molecules, which is the intensive quantity, and which comes closest to being a constant for all substances at high temperatures"
  • "Furthermore (although at generally higher temperatures yet) internal vibrational degrees of freedom also may become active."
  • "At higher temperatures, however, nitrogen gas gains two more degrees of internal freedom, as the molecule is excited into higher vibrational modes which store thermal energy"

There seems to be a lot of use of the qualitative phrase "high temperature". But high temperatures in cryogenics are a coupe milliKelvin, and high temperatures in plasma physics are tens of thousands of Kelvin. The example of HCl is given (4165 K) which helps to provide a sense of scale to what is meant by the phrase "high temperature", but in general I would encourage use of more specific terms throughout the article.--JB Gnome (talk) 22:23, 21 November 2010 (UTC)[reply]

You have hit on a critical point which is that high temperature is really a relative term. In heat capacities, it is used relative to the energy level spacing of whatever degree of freedom you're trying to excite. This will be made clearer. Mgibby5 (talk) 20:46, 11 July 2015 (UTC)[reply]

Symbol consistency[edit]

It's been my experience that heat capacity is capital C, and specific heat capacity is lower case c. The article mixes them. If this is a general convention, could we update the article? One particular example is the table of specific heat capacities; the first column should certainly be lower case c to agree with the article. I've gone ahead and changed that, but I'd appreciate if someone more knowledgeable could confirm my hunch. Khakiandmauve (talk) 19:26, 24 November 2010 (UTC)[reply]

value error[edit]

The heat capacity for water(ice) was inappropriately low. Clearly the molar mass of water does not change between phases. --Hansonrstolaf (talk) 23:42, 6 January 2011 (UTC)[reply]

Just so no one gets confused, the molar mass is not what dictates the heat capacity, and the heat capacity of ice at -10 C is around 2 kJ/kg/K. See: http://www.engineeringtoolbox.com/ice-thermal-properties-d_576.html Mgibby5 (talk) 20:54, 11 July 2015 (UTC)[reply]

Also: the given value for wood (in the building materials table) seems about 10x too large. - please fix. PB 20130605 — Preceding unsigned comment added by 81.151.170.144 (talk) 09:15, 5 June 2013 (UTC)[reply]

This has been resolved, or was already accurate in the first place. In looking through data, for wood, I discovered there was a wider range, and I have changed the table to reflect this. Mgibby5 (talk) 20:54, 11 July 2015 (UTC)[reply]

Sign of infinitesimal work term[edit]

In the section Thermodynamic relations and definition of heat capacity, User:65.183.8.194 changed the sign of the infinitesimal work term in the equation dU = dQ (was plus, now minus) dW. We never really define anywhere what convention we use, but I think it's typical to define work done on the system as positive, and work done by the system as negative; therefore, the proper form would be dU = dQ + dW. Thoughts? Khakiandmauve (talk) 21:44, 18 April 2011 (UTC)[reply]

(cv) or (cV)?[edit]

Take a look at the equation following the text: 'this equation reduces simply to Mayer's relation'. it is not clear to me why there is a lowercase subscript (v) in that equation as opposed to the usual uppercase (V).|Moemin05 (talk) 13:40, 20 April 2011 (UTC)[reply]

I think it's supposed to be Cp - Cv = R. Should be written Cv though. --AndreCharles (talk) 03:28, 3 November 2016 (UTC)[reply]

Sequence of article[edit]

I've just reverted this edit.

For a general audience, it is much more relevant to discuss first what heat capacity means at all in macroscopic terms -- even with the odd equation -- before getting into the microscopic physics of why some substances have the particular values of heat capacity they do at particular ranges.

This distinction -- discussing what the concept means at all first, and what can be said about it macroscopically in full generality -- before getting into any discussion of particular microscopic systems, i.e. the article as it was in its existing form, seems to me by far the more sensible presentation.

But of course, as after any instance of WP:BRD, further discussion and perspectives would be valuable and welcome. Jheald (talk) 18:43, 20 May 2011 (UTC)[reply]

Layman explanation[edit]

If possible please add a sentence in the introduction to explain what this is for people (including myself) without a ton of scientific background. My question that caused me to come to this article was "is this directly related to thermal conductivity, or is it a different term for thermal conductivity, or is it possibly inversely related to thermal conductivity? Or am I in the wrong place all together?" - so I really think the minor change, done by someone competant in the subject could help a lot of people. thanks - Sam

I agree that the lead as it stands now is more technical than it needs to be. Heat capacity is a simple concept that people can grasp without a scientific background. Still all of the article is written in a way that is only accessible for someone with such a background. Could we at least have one sentence early on describing heat capacity in a non-technical way. The least technical sentence is the first one and now reads
"Heat capacity, or thermal capacity, is a measurable physical quantity equal to the ratio of the heat added to (or subtracted from) an object to the resulting temperature change."
Could this maybe be changed into something more like "Heat capacity, or thermal capacity measures an objects resistance to temperature change. More specifically it is the ratio of the heat added to (or subtracted from) an object to the resulting change in temperature.". Ulflund (talk) 04:57, 13 November 2014 (UTC)[reply]
I agree that the third and fourth paragraphs are a bit too technical for the intro and could maybe moved into some of the subsections below. I'll work on this a bit and think of how it can best be accomplished. Maybe some other editors have some input on this topic as well. However, I wouldn't want the first sentence to be changed very much. In Ulflund's proposed sentence, I believe that it would be incomplete without including "...resulting change in temperature upon the addition of energy in the form of heat." In this case, it says essentially the same thing as what's already there except without using the word ratio. The general reader should be able to understand the concept of a ratio, which makes the definition a bit more rigorous. JCMPC (talk) 14:33, 13 November 2014 (UTC)[reply]
Yes, the general reader should know what a ratio is, but it requires some focus to read for a layman when the numerator and denominator are 10 words apart. I'm not sure but I would guess that the majority of the readers of this article are like the OP, with no scientific background, wanting only an easily grasped rough idea of what heat capacity is. The readers who want a strict definition probably don't mind reading eight extra words before getting to this definition. According to WP:Technical "Every reasonable attempt should be made to ensure that material is presented in the most widely understandable manner possible." Ulflund (talk) 16:36, 13 November 2014 (UTC)[reply]
Agreed the wording is awkward. I think it would help to have a separate sentence introduce heat and temperature change. That way the sentence about the ratio can be more concise. And definitely leave out the "or subtracted from" phrase. This isn't an insurance contract. Possibly: "Heat capacity is a measure of the amount of heat required to raise the temperature of an object by a given amount. It is defined as the ratio of the added heat to the change in temperature." Spiel496 (talk) 21:33, 13 November 2014 (UTC)[reply]
Spiel496, your proposed revision looks good to me. It brings the numerator and denominator closer together in the statement. Not to get too technical, but what about the temperature dependence of heat capacity. Could we maybe add an additional sentence to clarify? Although, it would need to be put in place rather well to make it easily understandable by the layperson. Also, what do others think of moving paragraphs 3 and 4 into appropriate section below. I don't see the need to mention quantum or definitions of energy in the lede. JCMPC (talk) 01:36, 14 November 2014 (UTC)[reply]
I prefer the original version over the one proposed by Spiel496 due to accuracy. To say that it is the heat required to do something indicates that it should have the dimension of energy. I'm for moving paragraphs 3 and 4, but I'm not why the temperature dependence is so important. Almost everything depends on temperature. Maybe we could give examples of the heat capacity of e.g. water or a diatomic gas? Ulflund (talk) 07:35, 14 November 2014 (UTC)[reply]
If my wording changed the technical content in any way, that was unintentional. I was just trying to say "heat goes in, temperature goes up" in a slightly different way. Strictly speaking, the heat is not "required", I admit. However, I am at a loss as to the objection that heat should have the dimensions of energy. Doesn't it? Units of heat are Joules. Joules are energy. Spiel496 (talk) 03:42, 16 November 2014 (UTC)[reply]
Yes, but heat capacity has unit J/K. Ulflund (talk) 04:56, 16 November 2014 (UTC)[reply]
Oh, I get what you're saying.Spiel496 (talk) 17:19, 16 November 2014 (UTC)[reply]
Ulflund, just to clarify, to which original version do you refer? JCMPC (talk) 16:50, 17 November 2014 (UTC)[reply]
The version that is still in the article (although I would remove the word "measureable" and the paranthesis). Ulflund (talk) 05:54, 18 November 2014 (UTC)[reply]
How about: "Heat capacity, or thermal capacity, is a physical property of an object, equal to the ratio of heat to temperature change, when heat is added or removed." Spiel496 (talk) 00:52, 19 November 2014 (UTC)[reply]
I'm ok with this (although I still prefer my original suggestion). I would link physical property and temperature. Ulflund (talk) 03:02, 19 November 2014 (UTC)[reply]
I'm OK with this one too, but it still involves the use of the word ratio, which I thought you were initially opposed to. What about "...a physical property of an object that describes its capacity to absorb heat without undergoing a large change in temperature. The heat capacity is measured as a ratio of heat absorbed/released by the object to the temperature change." JCMPC (talk) 20:52, 23 November 2014 (UTC)[reply]

specific heat is an intensive property[edit]

it says extensive. — Preceding unsigned comment added by 131.212.84.146 (talk) 20:51, 6 June 2011 (UTC)[reply]

water: specific heat capacity > 3R[edit]

-> more degrees of freedom or Dulong-Petit not the upper boundary or else ?

The molar mass of water is (2*1,008+15,999)g/mol = 18,015 g/mol. In 1g water are therefore 2*0,055509 mol H-atoms(!) und 0,055509 mol O-atoms.

The maximum value -according Dulong-Petit law- of the specific heat capacity of liquid water is therefore 2*0,055509g/mol*3R +0,055509g/mol*3R = 0,499958g/mol * 8,3145 J/molK =4,154 J/gK. But the real value is 4,18-4,19 J/gK. It's 0,7% bigger!(not much but well above the error boundaries)

What is the explanation of this? (31 October 2006) < I had posted that question 5 years ago. --Bgm2011 (talk) 11:16, 28 August 2011 (UTC)[reply]

I'm very glad to answer your question. I don't know. (Mark Twain). At a guess, since the 3R limit is derived by assuming strictly that ONLY kinetic and vibration-potential energies of atoms are being excited by heat, and these are the only degrees of freedom available, whenever anything goes above 3R/mole of atoms, that is a sign that some other degree of freedom is available to store thermal energy, such as the molecular electronic excitation noted that contributes to the heat capacity of nitric oxide. In liquid water there may be a little of that. SBHarris 23:21, 5 February 2012 (UTC)[reply]

Please could we have basic practical information that we can actually use ?[edit]

I have a reasonable grasp of science in general, but have forgotten the details of what I learned at school.

I came to this page wanting to discover how to calculate how long my electric kettle should take to boil a cup of water.

The vast majority of the discussion is extremely academic, and having read (or tried to !) through the whole article, I am fascinated by the topic, but I am no nearer to finding out how to do my calculation.

Would it be possible to include a simplified section, giving the basic information, which would be useful to ordinary people who do not have a degree in physical science ?

I think I should be able to read a short way into this article and find a practical formula that I can easily use. Darkman101 (talk) 11:41, 5 February 2012 (UTC)[reply]

The energy delivered by the electric kettle should equal the energy needed to heat up your cup of water. Ask your specific question on Wikipedia:Reference desk/Science rather than expecting this article to answer it. Bo Jacoby (talk) 19:46, 5 February 2012 (UTC).[reply]
The article does have a practical formula, C = Q / ΔT, and we have a data table at the end giving the value of C for water. So that should tell you how much heat per kilogram you need to supply your desired increase in temperature; from which it should be easy enough to work out long you need to run your kettle to supply that heat.
So I think the material needed is in the article. But if people aren't picking up in our article that this is how to use heat capacities, then maybe the article's signposting needs some thinking about. Jheald (talk) 20:22, 5 February 2012 (UTC)[reply]
You're going to have to measure it anyway, since you don't know the efficiency of your kettle. In this example from a consumer, it was 81%. But he had to do the experiment directly anyway, so it didn't help him. If you assumed 100% efficiency, you could have guesstimated the problem and only been 20% off. [1] SBHarris 23:55, 5 February 2012 (UTC)[reply]

Following on JHeald's comment, should we perhaps actually stick C = Q / ΔT in the lede in that form (appropriate for inclusion in a normal line of text), with an explanation of the symbols? I know the text says this in words, but sometimes a formula helps, if it doesn't screw up spacing. WP is not a popular book, in which it is said that every formula will decrease sales by half. Usually we try to keep formulas out of ledes, but if a formula can go into a text line, we might make an exception here. What say you all? SBHarris 20:12, 6 February 2012 (UTC)[reply]

incorrect link[edit]

The link to the outdated caloric theory instead takes you to the appliance brand — Preceding unsigned comment added by 58.172.92.222 (talk) 07:48, 29 July 2012 (UTC)[reply]

I have now corrected this. Ulflund (talk) 06:09, 30 July 2012 (UTC)[reply]

Factors that affect specific heat capacity Mistake[edit]

"The kinetic energy of substance particles is the only one of the many possible degrees of freedom which manifests as temperature change, and thus the larger the number of degrees of freedom available to the particles of a substance other than kinetic energy, the larger will be the specific heat capacity for the substance." This statement is incorrect, and "kinetic energy" should be replaced with "translation". The kinetic energy of molecules includes all of the motion of atoms (translation, vibration, rotation, stretching, compression, etc.)--El Zarco 03:41, 2 January 2013 (UTC) — Preceding unsigned comment added by ElZarco (talk • contribs)

Yes, it's translational kinetic energy. Will change. SBHarris 04:52, 2 January 2013 (UTC)[reply]


Is it necessary to continue the dubious habit of refering to the Dulong-Petit "Law" as a legitimate physical law? Their hypothesis is patently inaccurate in the best of cases. Parroting the rote habit of proclaiming that it correctly predicts the heat capacity of solids, as done here, is completely without merit and disregards the complexity of the problem. Would anyone care to prove otherwise?Wikibearwithme (talk) 02:30, 7 January 2016 (UTC)[reply]

The article seem very bad to me. But regarding the Dulon-Petit law it's accurate. If fit too well with empirical value to be anecdotic. The article in french is mutch better.--70.81.186.183 (talk) 16:11, 22 October 2016 (UTC)[reply]

Units[edit]

Hi, Just wanted to flag the fact that in the first paragraph: "In the International System of Units (SI), heat capacity is expressed in units of joule(s) (J) per kelvin (K)." I didn't want to edit it because I wasn't completely sure, but isn't it joules per GRAM per kelvin? The formula q = mCΔT can be arranged as C = q/(mΔT)... or C = joules per gram per kelvin.

Thanks! A Wiki Amateur

Bubbathemonkey (talk) 12:00, 7 April 2013 (UTC)[reply]

The formula is correct for heat capacity. You divide by mass to get mass-specific heat capacity (sometimes just called specific heat capacity). A different thing. Read the article, please. Learn something. SBHarris 18:48, 7 April 2013 (UTC)[reply]
The phrase "of a substance" sounds like the language one would use when explaining specific heat capacity, rather than just "heat capacity". Would it be more accurate if we said "...heat required to change the temperature of an object by a given amount"? Spiel496 (talk) 04:18, 8 April 2013 (UTC)[reply]
To put it another way, when someone reads "...heat required to change the temperature of a substance..." their mind jumps to "heat capacity of a substance", which sounds like a material property, i.e. "specific heat capacity", even though that's not what is being defined. Spiel496 (talk) 04:37, 8 April 2013 (UTC)[reply]
Good point. Should say "body" or object or something. Indeed. SBHarris 07:38, 8 April 2013 (UTC)[reply]
Thanks - I learnt something today! I read the units as J/K, then skipped down to the section on units. I had assumed 'specific heat capacity' and 'heat capacity' were the same thing - I didn't even look at the second paragraph. Maybe we could add a 'distinguish' for people like me? Thanks again! Bubbathemonkey (talk) 12:23, 9 April 2013 (UTC)[reply]
We usually have "distinguish" for other articles and sometime ago it was decided to cover both closely related terms in just one (this one). I'll try to add some distinguishing language early to make this more clear. But really, dude, what can you expect if you don't even read paragraph TWO? SBHarris 15:54, 9 April 2013 (UTC)[reply]
Hi, I had exactly the same issue, and even created a wikipedia account for that. I often use it just to look up units. I think an information which is so likely to be missed should be in the first paragraph, or at least clearer in the section "units" below. I only looked at units, and only found it when seeing the one in the "Measurement" section even further down. S 14:59, 3 October 2017 (UTC)[reply]


Hi, This page kept getting me confused over and over again, and finally I understand where my confusion comes from, yet I didn't want to edit directly. I have no formal physics or chemistry education, and even more to my disgrace, this is my first Wikipedia edit. Therefore I chose to add my remarks to the discussion with hopes that someone more knowledgeable can verify my claims and fix the errors.

In the introduction (first paragraph), heat capacity is described and then the units are for specific heat capacity. It is also inconsistent with the dimensional form in the same paragraph (T−2L2MΘ−1), which I believe is correct for heat capacity, but not for specific heat capacity. I tracked this error to a recent [2]. It was even more confusing because later, in Extensive Properties, it is defined correctly, but nowhere else are **specific heat capacity**'s units described. Oranjax (talk) 12:36, 13 December 2016 (UTC)[reply]

Heat capacity is the total kinetic energy of matter?[edit]

Kbrose, what the devil are you thinking? The statement is true for helium gas, but the reason a mole of iron metal has twice the heat capacity of a mole of helium is very simple: half the thermal energy in iron is stored as potential energy of vibration and that doubles the degrees of freedom. So why are you screwing up the lede? I don't have the wrong physics. You do. When I melt ice I do not increase temperature therefore do not increase mean particle kinetic energy. But of course one must increase thermal energy by putting in heat. Therefore thermal energy in this case is in no sense kinetic energy. QED. SBHarris 08:35, 14 May 2013 (UTC)[reply]

First off, Kbrose didn't say that heat capacity is the total kinetic energy, but rather thermal energy is. Second, is one of you (Kbrose or Sbharris) necessarily wrong, or are we just talking about alternative definitions? I can define "XYZZY" to be the "total kinetic energy of a system". It doesn't become right or wrong physics until XYZZY is linked to an experimental result. Self-consistency is the more relevant issue here. I notice that Kbrose's definition is inconsistent with the article thermal energy, which defines the term to include potential energy.
If we do define "thermal energy" to be only the kinetic energy, then, for example, a 1 kg bag of ice water would have the same thermal energy even after most of the ice has melted. Is that definition useful? Is it the conventional one in textbooks? Spiel496 (talk) 01:26, 15 May 2013 (UTC)[reply]
The short answer to your last two questions is no and no.

I will admit that there are subtle problems defining "thermal energy," due mainly to the fact that once heat has flowed into an object, it's a little tricky to say what happened to it. Certainly the heat increases an object's total energy (internal energy), but not all of it sometimes can be recovered again as heat by reversing the thermal contact and cooling, for example if some of it was used to do work (if the object expands and does work, then cooling it again might not recover that energy if the environment doesn't cooperate by compressing it again). So in that case, how can we continue to label that heat energy "thermal"? However, in the absense of PV work problems, most texts do define thermal energy in terms of heat capacity, simply as integral ∫ C(T)dT where C is the heat capacity and T is the temperature. So "thermal energy" is what we used to call "heat content" before we got serious and stopped calling heat "heat" after it had stopped flowing. In that case, half of the thermal energy in solids is not kinetic, but exists as potential energies (again ignoring work).

It might be better not to refer to thermal energy at all in the lede (what is the case now), but if we do, we should surely not use a definition that NOBODY uses. Thermal energy is used in engineering texts now in place of "heat content". The reason being that thermal energy is more or less conserved (in the absense of PV work) in certain heat conduction problems (mostly those in solids and liquids where there is little volume change) SBHarris 02:56, 15 May 2013 (UTC)[reply]

The introduction contains a fundamental error. Needs revision.[edit]

<Quote> while heat is the transfer of thermal energy across a system boundary into the body or from the body to the environment. <unquote> This is risible. Heat is NOT the transfer of ANYTHING. Was this written by a non-fluent English speaker? It is possible that the author meant "to heat" is the transfer of ..." , but why s/he would confuse a noun with a verb is a mystery to me. Nor is it useful. If someone is unable to differentiate things from processes, I suggest they refrain from editing. Heat is a thing, it is a type of energy. According to my old Physics textbook (Physics Pts I & II, c 1966, Halliday & Resnick, pg. 640) "Freeman Dyson, in an article* "What is Heat?" writes:'Heat is disordered energy.' [*Scientific American 1954]". Same text, pg 545 "Heat is that which is transferred between a system and its surroundings as a result of temperature differences only." and "..heat is a form of energy rather than a [material] substance." It is true that heat occurs due to the transfer of energy. But claiming that the transfer of "thermal" energy is heat is mangled logic: thermal energy IS heat. Claiming that the transfer of the thing is the thing would have any Freshman logician tossed out on his/her ear, and rightly so. Please note that the Wikipedia article on Heat makes the same profound blunder, I suspect the same cabal is responsible. Heat energy is not heat flow. If the point is that heat energy is always associated with the transfer of energy, that is fine. Heat is energy. Transfer is not energy. 173.189.78.236 (talk) 01:33, 18 May 2013 (UTC)[reply]

For reasons too complicated to go into here, thermodynamics now defines heat energy only in transfer. Once it stops it is no longer heat, just as a stopped photon is no longer a photon. When heat stops it becomes internal energy. Not thermal energy, as this term is poorly defined, and in any case heat often becomes other types of energy after stopping, like work or chemical energy. There's no point in defining heat as the transfer of a thing when you cannot rigorously define the "thing" as well as you can define heat! Heat is not just energy, though it has energy units. Incremental heating δQ also requires a specific associated entropy change dS = δQ/T. I wish that were true of some quantity we could define as "thermal energy" once it had been transferred, but it isn't. SBHarris 19:04, 23 May 2013 (UTC)[reply]
I agree that the the intro (lead section) needs polishing, and I'm impressed by the writing skill both the above authors and especially the physics textbook author. I think that discussion is so fascinating and BASIC, a short version might belong in the intro? To me "writing skill" is "communication skill." Effective communication is not defined by the words used, but by the rate or quantity of the transfer of desired/useful information. For example...how is the meaning of "red" best communicated? First we need to know what is desired/useful to the reader. A list of true abstract things about red might impress our teachers and peers about how knowledgeable we are, (effectively reciting the answers to an imaginary test,) but will typically do little to explain red to the unknowledgeable. Probably first reciting the wavelength range of 740–620 nanometres, or saying it's the color of light with the longest waves that humans can see, would both be true, but wrong, since there is no "red" without experience/perception, at least for most people. I would first define it with an example such as blood or ripe strawberries, probably both. And Cambridge dictionary agrees. I think heat capacity is such a thing, like Ohm's Law, one can have a gut feeling for the workings of the reality they describe, and numbers for many people are only useful for confirmation or added precision.
I really think the lead needs some examples to pin down all those abstractions. Perhaps water Vs iron Vs...marble?? A whole paragraph! See also: Wikipedia:Manual of Style (lead section). Beware of too many details, and welcome sentences that start with "generally" to portray concepts and general rules, (overviews,)—which is what the lead is all about. Cumbersome rules that enclose the rarities & "exceptions" and the extremely nit-picky or extremely technical, belong in the body where they can be explored in depth. Also beware, I think the lead might already be too technical, have too much jargon for the required "general reader." Thanks!
--69.110.90.19 (talk) 09:23, 27 May 2013 (UTC)Doug Bashford[reply]

Values for water[edit]

Values in the table and in section 'Measurement of heat capacity' are not in agreement. —DIV (137.111.13.36 (talk) 02:54, 30 October 2013 (UTC))[reply]

The table claims the mass heat capacity is the same at 25 deg C and 100 deg C, which disagrees with the page Properties of water. Chemical Engineer (talk) 22:37, 30 December 2013 (UTC)[reply]

Definition of Heat Capacity[edit]

Can anyone explain to me why we have the current definition of heat capacity, C=ΔQ/ΔT? I know that some texts use this or a similar expression as the definition of heat capacity, but I've never liked it. To me, the Δ's imply a discrete change and that heat capacities are constants when, in fact, they are highly temperature dependent. Also, I don't like the ΔQ term as it implies that Q (heat) is a state function, which it is not. Would people be OK with changing the definition to C=δq/dT. Changing to an infinitesimal change helps solve the problem of C=C(T) an the use of δ implies an inexact differential, which is the same notation used in most of the article. JCMPC (talk) 20:23, 22 January 2014 (UTC)[reply]

You make a good point. Even if we don't go all the way to "C=δq/dT" we should at least say "Q/ΔT" so that heat doesn't look like a state function. Spiel496 (talk) 02:24, 23 January 2014 (UTC)[reply]
I wholeheartedly agree. Why has the definition not been changed to "Q/ΔT", yet? Are there any defenders or the current formulation who would like to weigh in? — Preceding unsigned comment added by 2A01:C50F:9D4A:5500:C884:7819:1534:5966 (talk) 12:17, 19 February 2021 (UTC)[reply]

Very good point. The quantum expression of the heat capacity is mutch more elusive. I try to reform the molecular orbital theory and it's almost impossible to make an intelligible text about it. It must be filled with simple unnacurate physical equation because I think it's out of the realm of humans logic and something that only computer can do. The expression is $$C \ind(V,m)= \frac(1,2)\times(3 + v_R^* + v_V^*)R$$ 70.81.186.183 (talk) 16:21, 22 October 2016 (UTC)[reply]

Error in theory part concerning degrees of freedom[edit]

Hi there, there is a mistake in the Theory part, stating that "At higher temperatures, however, nitrogen gas gains two more degrees of internal freedom, as the molecule is excited into higher vibrational modes that store thermal energy. Now the bond is contributing heat capacity, and is contributing more than if the atoms were not bonded."

This is not true: Nitrogen gains only ONE vibrational degree of freedom. Any molecule consisting of N atoms can under no circumstances have more than 3N degrees of freedom. For a diatomic molecule, that is 6 degrees. Three of translation, two of rotation, and one vibrational degree of freedom. There is only one vibrational mode, the stretching mode of the N-N triple bond. Best, Naclador (talk) 12:44, 21 May 2014 (UTC)[reply]

I think this article is making the correct point about the contribution to heat capacity, namely that vibrational modes have two "receptacles" to store energy, i.e. potential and kinetic energy. However, it says, I believe wrongly, that a vibration mode represents two degrees of freedom. According to Degrees_of_freedom_(physics_and_chemistry)#Degrees_of_freedom_of_gas_molecules a diatomic molecule has one vibrational degree of freedom for a total of six, not seven. Spiel496 (talk) 18:23, 11 June 2014 (UTC)[reply]
Yes, both comments above are right. Only one additional degree of freedom is gained, because the total cannot be more than 6 for 2 atoms. However, the additional vibrational degree of freedom contributes R-per-mole extra heat capacity, due to 2* 1/2 R contribution per excited vibrational mode. I've fixed the text to reflect the difference. SBHarris 03:27, 4 March 2016 (UTC)[reply]

Regarding pressure[edit]

The last paragraph of "Measurement of heat capacity" states that it is important to know the pressure when measuring heat capacity in liquids and gases. It might be redundant, but is it worth mentioning that this only applies to compressible liquids? ..if this is correct, of course. 82.147.45.122 (talk) 04:35, 29 August 2014 (UTC) Peter Heiberg, Norway[reply]

Intensive vs. Extensive[edit]

Currently, the article seems to be inconsistent (or at least unclear) as to whether or not heat capacity is intensive or extensive. In the lead, the article states that heat capacity has units of J/K, which implies the article discusses the extensive quantity, and the section "Extensive and intensive quantities" seems to imply that C is extensive; however, many of the math in the bottom half of the article uses a notation that implies that heat capacity is an intensive quantity. Would people mind editing the article such that every time it uses "C", it strictly means the extensive quantity? If so, lots of the math will have to be edited by either including the number of moles or by changing the notation to explicitly use molar quantities. For example, could we use either or perhaps to indicate when the molar constant volume heat capacity is being used? JCMPC (talk) 23:52, 18 October 2014 (UTC)[reply]

The article covers both the extensive property heat capacity (of an object) and the intensive properties specific heat capacity and molar heat capacity (of a substance). The "extensive and intensive quantities" section uses the notation that capital C without subscripts means the heat capacity of an object, lower case c means per unit mass, and uses the subscript "mol" for molar heat capacity. Most other parts of the article seem to use the subscript ",m" to mean the molar quantity (e.g. C_{V,m} for molar heat capacity at constant volume), not sure if they are consistent about C vs c or indeed about including the "m". I think the rest of the world is equally inconsistent, but I agree it would be good to pick one convention for this article and stick with it! Djr32 (talk) 13:12, 19 October 2014 (UTC)[reply]
If I recall correctly, there used to be articles on both "heat capacity" (extensive) and "specific heat capacity" (intensive). That was recognized as redundant, so they were merged into "specific heat capacity". Then another editor came along and saw the lack of a "heat capacity" article as a travesty, and the article was renamed. The result was a hodgepodge of terminology, which hasn't been completely ironed out. So, yes, go forth if you're inspired. Spiel496 (talk) 15:33, 19 October 2014 (UTC)[reply]
I believe that you're correct. Personally, I'm a fan of having two articles, where a shorter specific heat article would be a more pragmatic approach for "everyday use" and a longer heat capacity article could go into all of the detail; but that's just opinion. For this particular issue, where I was especially confused was in the section on the relationship between heat capacities for an ideal gas. Based on the form of the equations, they clearly all use intensive heat capacities and intensive volume; however, they are not explicitly labeled as such. JCMPC (talk) 16:37, 19 October 2014 (UTC)[reply]
I meant to ask in my last post, would people prefer to label all relevant intensive terms, or keep the terms extensive by including the number of moles? JCMPC (talk) 16:40, 19 October 2014 (UTC)[reply]
I decided that I might not have the time to edit later and didn't want to end up forgetting this one, so I went ahead and made the changes. If people would prefer using intensive quantities, just let me know and I can change them over. JCMPC (talk) 16:50, 19 October 2014 (UTC)[reply]
I think there should be one article not several, though understanding the history does explain why the existing article is a bit of a mess. To me, heat capacity per molecule is the fundamental thing, with heat capacity per mole, per unit mass or of a macroscopic object coming from it. Even in everyday use, for most practical purposes our thermodynamic systems contain fluids like water or air, i.e. how much electricity does it take to boil a kettle per litre of water I put in it?. Hence I'd prefer to use intensive terms wherever possible, rather than having extraneous ns.


Simple professional opinion[edit]

This article is useless as a body of information to which students can be pointed for their self-training. This is stated emphatically, for the extent to which the editors here have allowed the article to become a depository for cribbed information appearing without any attribution, and of similar long descriptions of phenomena not tied to any authoritative, scholarly underpinnings. To appreciate the magnitude of the issue here, consider the following copy of the current text of the entire article, where I have redacted any paragraph or part of paragraph where a citation DOES appear. That is, the following is the current body of unverifiable material in the article:

Extended content

Background[edit] Today, the notion of the caloric has been replaced by the notion of a system's internal energy. That is, heat is no longer considered a fluid; rather, heat is a transfer of disordered energy. Nevertheless, at least in English, the term "heat capacity" survives. In some other languages, the term thermal capacity is preferred, and it is also sometimes used in English.

Older units and Imperial units[edit] An older unit of heat is the kilogram-calorie (Cal), originally defined as the energy required to raise the temperature of one kilogram of water by one degree Celsius, typically from 14.5 to 15.5 °C. The specific average heat capacity of water on this scale would therefore be exactly 1 Cal/(C°·kg). However, due to the temperature-dependence of the specific heat, a large number of different definitions of the calorie came into being. Whilst once it was very prevalent, especially its smaller cgs variant the gram-calorie (cal), defined so that the specific heat of water would be 1 cal/(K·g), in most fields the use of the calorie is now archaic.

In the United States other units of measure for heat capacity may be quoted in disciplines such as construction, civil engineering, and chemical engineering. A still common system is the English Engineering Units in which the mass reference is pound mass and the temperature is specified in degrees Fahrenheit or Rankine. One (rare) unit of heat is the pound calorie (lb-cal), defined as the amount of heat required to raise the temperature of one pound of water by one degree Celsius. On this scale the specific heat of water would be 1 lb-cal/(K·lb). More common is the British thermal unit, the standard unit of heat in the U.S. construction industry. This is defined such that the specific heat of water is 1 BTU/(F°·lb).

Extensive and intensive quantities[edit] An object's heat capacity (symbol C) is defined as the ratio of the amount of heat energy transferred to an object and the resulting increase in temperature of the object, 

C \equiv \frac{ Q}{\Delta T},

assuming that the temperature range is sufficiently small so that the heat capacity is constant.

More generally, because heat capacity does depend upon temperature, it should be written as 

C (T) \equiv \frac{\delta Q}{d T},

where the symbol δ is used to imply that heat is a path function.

In the International System of Units, heat capacity has the unit joules per kelvin.

Heat capacity is an extensive property, meaning it depends on the extent or size of the physical system in question. A sample containing twice the amount of substance as another sample requires the transfer of twice the amount of heat ( Q ) to achieve the same change in temperature ( \Delta T ).

. . .

In chemistry, heat capacity is often specified relative to one mole, the unit of amount of substance, and is called the molar heat capacity. It has the unit [C_\mathrm{mol}] =\mathrm{\tfrac{J}{mol \cdot K}}.

For some considerations it is useful to specify the volume-specific heat capacity, commonly called volumetric heat capacity, which is the heat capacity per unit volume and has SI units [s] = \mathrm{\tfrac{J}{m^{3} \cdot K}}. This is used almost exclusively for liquids and solids, since for gases it may be confused with specific heat capacity at constant volume.

Measurement of heat capacity[edit] The heat capacity of most systems is not a constant. Rather, it depends on the state variables of the thermodynamic system under study. In particular it is dependent on temperature itself, as well as on the pressure and the volume of the system.

Different measurements of heat capacity can therefore be performed, most commonly either at constant pressure or at constant volume. The values thus measured are usually sub scripted (by p and V, respectively) to indicate the definition. Gases and liquids are typically also measured at constant volume. Measurements under constant pressure produce larger values than those at constant volume because the constant pressure values also include heat energy that is used to do work to expand the substance against the constant pressure as its temperature increases.

. . .

Calculation from first principles[edit] The path integral Monte Carlo method is a numerical approach for determining the values of heat capacity, based on quantum dynamical principles. However, good approximations can be made for gases in many states using simpler methods outlined below. For many solids composed of relatively heavy atoms (atomic number > iron), at non-cryogenic temperatures, the heat capacity at room temperature approaches 3R = 24.94 joules per kelvin per mole of atoms (Dulong–Petit law, R is the gas constant). Low temperature approximations for both gases and solids at temperatures less than their characteristic Einstein temperatures or Debye temperatures can be made by the methods of Einstein and Debye discussed below.

Thermodynamic relations and definition of heat capacity [edit] The internal energy of a closed system changes either by adding heat to the system or by the system performing work. Written mathematically we have

Δesystem = ein - eout Or

{\ \mathrm{d}U = \delta Q + \delta W }. For work as a result of an increase of the system volume we may write,

{\ \mathrm{d}U = \delta Q - P\mathrm{d}V }. If the heat is added at constant volume, then the second term of this relation vanishes and one readily obtains

\left(\frac{\partial U}{\partial T}\right)_V=\left(\frac{\partial Q}{\partial T}\right)_V=C_V. This defines the heat capacity at constant volume, CV, which is also related to changes in internal energy. Another useful quantity is the heat capacity at constant pressure, CP. This quantity refers to the change in the enthalpy of the system, which is given by

{\ H = U + PV }. A small change in the enthalpy can be expressed as

{\ \mathrm{d}H = \delta Q + V \mathrm{d}P }, and therefore, at constant pressure, we have

\left(\frac{\partial H}{\partial T}\right)_P=\left(\frac{\partial Q}{\partial T}\right)_P=C_P. These two equations:

\left(\frac{\partial U}{\partial T}\right)_V=\left(\frac{\partial Q}{\partial T}\right)_V=C_V. \left(\frac{\partial H}{\partial T}\right)_P=\left(\frac{\partial Q}{\partial T}\right)_P=C_P. are property relations and are therefore independent of the type of process. ...

Relation between heat capacities[edit] Main article: Relations between heat capacities Measuring the heat capacity, sometimes referred to as specific heat, at constant volume can be prohibitively difficult for liquids and solids. That is, small temperature changes typically require large pressures to maintain a liquid or solid at constant volume implying the containing vessel must be nearly rigid or at least very strong (see coefficient of thermal expansion and compressibility). Instead it is easier to measure the heat capacity at constant pressure (allowing the material to expand or contract freely) and solve for the heat capacity at constant volume using mathematical relationships derived from the basic thermodynamic laws. Starting from the fundamental thermodynamic relation one can show

C_P - C_V = T \left(\frac{\partial P}{\partial T}\right)_{V,n} \left(\frac{\partial V}{\partial T}\right)_{P,n} where the partial derivatives are taken at constant volume and constant number of particles, and constant pressure and constant number of particles, respectively.

This can also be rewritten

C_P - C_V = V T\frac{\alpha^{2}}{\beta_T} where

\alpha is the coefficient of thermal expansion, \beta_T is the isothermal compressibility. The heat capacity ratio or adiabatic index is the ratio of the heat capacity at constant pressure to heat capacity at constant volume. It is sometimes also known as the isentropic expansion factor.

Ideal gas[edit]

. . . Redaction assumed single citation adequate for whole subsection.

Specific heat capacity[edit] The specific heat capacity of a material on a per mass basis is

c = {\partial C \over \partial m}, which in the absence of phase transitions is equivalent to

c = E_ m = {C \over m} = {C \over {\rho V}}, where

C is the heat capacity of a body made of the material in question, m is the mass of the body, V is the volume of the body, and \rho = \frac{m}{V} is the density of the material. For gases, and also for other materials under high pressures, there is need to distinguish between different boundary conditions for the processes under consideration (since values differ significantly between different conditions). Typical processes for which a heat capacity may be defined include isobaric (constant pressure, dP = 0) or isochoric (constant volume, dV = 0) processes. The corresponding specific heat capacities are expressed as

c_P = \left(\frac{\partial C}{\partial m}\right)_P, c_V = \left(\frac{\partial C}{\partial m}\right)_V. From the results of the previous section, dividing through by the mass gives the relation

c_P - c_V = \frac{\alpha^2 T}{\rho \beta_T}. A related parameter to c is CV^{-1}\,, the volumetric heat capacity. In engineering practice, c_V\, for solids or liquids often signifies a volumetric heat capacity, rather than a constant-volume one. In such cases, the mass-specific heat capacity (specific heat) is often explicitly written with the subscript m, as c_m\,. Of course, from the above relationships, for solids one writes 

c_m = \frac{C}{m} = \frac{c_{volumetric}}{\rho}.

For pure homogeneous chemical compounds with established molecular or molar mass or a molar quantity is established, heat capacity as an intensive property can be expressed on a per mole basis instead of a per mass basis by the following equations analogous to the per mass equations:

C_{P,m} = \left(\frac{\partial C}{\partial n}\right)_P = molar heat capacity at constant pressure C_{V,m} = \left(\frac{\partial C}{\partial n}\right)_V = molar heat capacity at constant volume where n is the number of moles in the body or thermodynamic system. One may refer to such a per mole quantity as molar heat capacity to distinguish it from specific heat capacity on a per mass basis.

Polytropic heat capacity[edit] The polytropic heat capacity is calculated at processes if all the thermodynamic properties (pressure, volume, temperature) change

C_{i,m} = \left(\frac{\partial C}{\partial n}\right) = molar heat capacity at polytropic process The most important polytropic processes run between the adiabatic and the isotherm functions, the polytropic index is between 1 and the adiabatic exponent (γ or κ)

Dimensionless heat capacity[edit]

. . . Redaction assumed single citation adequate for whole subsection.

Heat capacity at absolute zero[edit] From the definition of entropy

T \, dS=\delta Q\, the absolute entropy can be calculated by integrating from zero kelvins temperature to the final temperature Tf

S(T_f)=\int_{T=0}^{T_f} \frac{\delta Q}{T} =\int_0^{T_f} \frac{\delta Q}{dT}\frac{dT}{T} =\int_0^{T_f} C(T)\,\frac{dT}{T}. The heat capacity must be zero at zero temperature in order for the above integral not to yield an infinite absolute entropy, which would violate the third law of thermodynamics. One of the strengths of the Debye model is that (unlike the preceding Einstein model) it predicts the proper mathematical form of the approach of heat capacity toward zero, as absolute zero temperature is approached.

Negative heat capacity (stars)[edit]

. . . Redaction assumed citations adequate for whole subsection.

Theory of heat capacity[edit] Factors that affect specific heat capacity[edit]

. . .

For any given substance, the heat capacity of a body is directly proportional to the amount of substance it contains (measured in terms of mass or moles or volume). Doubling the amount of substance in a body doubles its heat capacity, etc.

However, when this effect has been corrected for, by dividing the heat capacity by the quantity of substance in a body, the resulting specific heat capacity is a function of the structure of the substance itself. In particular, it depends on the number of degrees of freedom that are available to the particles in the substance, each of which type of freedom allows substance particles to store energy. The translational kinetic energy of substance particles is only one of the many possible degrees of freedom which manifests as temperature change, and thus the larger the number of degrees of freedom available to the particles of a substance other than translational kinetic energy, the larger will be the specific heat capacity for the substance. For example, rotational kinetic energy of gas molecules stores heat energy in a way that increases heat capacity, since this energy does not contribute to temperature.

In addition, quantum effects require that whenever energy be stored in any mechanism associated with a bound system which confers a degree of freedom, it must be stored in certain minimal-sized deposits (quanta) of energy, or else not stored at all. Such effects limit the full ability of some degrees of freedom to store energy when their lowest energy storage quantum amount is not easily supplied at the average energy of particles at a given temperature. In general, for this reason, specific heat capacities tend to fall at lower temperatures where the average thermal energy available to each particle degree of freedom is smaller, and thermal energy storage begins to be limited by these quantum effects. Due to this process, as temperature falls toward absolute zero, so also does heat capacity.

Degrees of freedom[edit] Question book-new.svg This section does not cite any references or sources. Please help improve this section by adding citations to reliable sources. Unsourced material may be challenged and removed.

Main article: degrees of freedom (physics and chemistry) Molecules are quite different from the monatomic gases like helium and argon. With monatomic gases, thermal energy comprises only translational motions. Translational motions are ordinary, whole-body movements in 3D space whereby particles move about and exchange energy in collisions—like rubber balls in a vigorously shaken container (see animation here [16]). These simple movements in the three dimensions of space mean individual atoms have three translational degrees of freedom. A degree of freedom is any form of energy in which heat transferred into an object can be stored. This can be in translational kinetic energy, rotational kinetic energy, or other forms such as potential energy in vibrational modes. Only three translational degrees of freedom (corresponding to the three independent directions in space) are available for any individual atom, whether it is free, as a monatomic molecule, or bound into a polyatomic molecule.

As to rotation about an atom's axis (again, whether the atom is bound or free), its energy of rotation is proportional to the moment of inertia for the atom, which is extremely small compared to moments of inertia of collections of atoms. This is because almost all of the mass of a single atom is concentrated in its nucleus, which has a radius too small to give a significant moment of inertia. In contrast, the spacing of quantum energy levels for a rotating object is inversely proportional to its moment of inertia, and so this spacing becomes very large for objects with very small moments of inertia. For these reasons, the contribution from rotation of atoms on their axes is essentially zero in monatomic gases, because the energy spacing of the associated quantum levels is too large for significant thermal energy to be stored in rotation of systems with such small moments of inertia. For similar reasons, axial rotation around bonds joining atoms in diatomic gases (or along the linear axis in a linear molecule of any length) can also be neglected as a possible "degree of freedom" as well, since such rotation is similar to rotation of monatomic atoms, and so occurs about an axis with a moment of inertia too small to be able to store significant heat energy.

In polyatomic molecules, other rotational modes may become active, due to the much higher moments of inertia about certain axes which do not coincide with the linear axis of a linear molecule. These modes take the place of some translational degrees of freedom for individual atoms, since the atoms are moving in 3-D space, as the molecule rotates. The narrowing of quantum mechanically determined energy spacing between rotational states results from situations where atoms are rotating around an axis that does not connect them, and thus form an assembly that has a large moment of inertia. This small difference between energy states allows the kinetic energy of this type of rotational motion to store heat energy at ambient temperatures. Furthermore internal vibrational degrees of freedom also may become active (these are also a type of translation, as seen from the view of each atom). In summary, molecules are complex objects with a population of atoms that may move about within the molecule in a number of different ways (see animation at right), and each of these ways of moving is capable of storing energy if the temperature is sufficient.

The heat capacity of molecular substances (on a "per-atom" or atom-molar, basis) does not exceed the heat capacity of monatomic gases, unless vibrational modes are brought into play. The reason for this is that vibrational modes allow energy to be stored as potential energy in intra-atomic bonds in a molecule, which are not available to atoms in monatomic gases. Up to about twice as much energy (on a per-atom basis) per unit of temperature increase can be stored in a solid as in a monatomic gas, by this mechanism of storing energy in the potentials of interatomic bonds. This gives many solids about twice the atom-molar heat capacity at room temperature of monatomic gases.

However, quantum effects heavily affect the actual ratio at lower temperatures (i.e., much lower than the melting temperature of the solid), especially in solids with light and tightly bound atoms (e.g., beryllium metal or diamond). Polyatomic gases store intermediate amounts of energy, giving them a "per-atom" heat capacity that is between that of monatomic gases (3⁄2 R per mole of atoms, where R is the ideal gas constant), and the maximum of fully excited warmer solids (3 R per mole of atoms). For gases, heat capacity never falls below the minimum of 3⁄2 R per mole (of molecules), since the kinetic energy of gas molecules is always available to store at least this much thermal energy. However, at cryogenic temperatures in solids, heat capacity falls toward zero, as temperature approaches absolute zero.

Example of temperature-dependent specific heat capacity, in a diatomic gas[edit] To illustrate the role of various degrees of freedom in storing heat, we may consider nitrogen, a diatomic molecule that has five active degrees of freedom at room temperature: the three comprising translational motion plus two rotational degrees of freedom internally. Although the constant-volume molar heat capacity of nitrogen at this temperature is five-thirds that of monatomic gases, on a per-mole of atoms basis, it is five-sixths that of a monatomic gas. The reason for this is the loss of a degree of freedom due to the bond when it does not allow storage of thermal energy. Two separate nitrogen atoms would have a total of six degrees of freedom—the three translational degrees of freedom of each atom. When the atoms are bonded the molecule will still only have three translational degrees of freedom, as the two atoms in the molecule move as one. However, the molecule cannot be treated as a point object, and the moment of inertia has increased sufficiently about two axes to allow two rotational degrees of freedom to be active at room temperature to give five degrees of freedom. The moment of inertia about the third axis remains small, as this is the axis passing through the centres of the two atoms, and so is similar to the small moment of inertia for atoms of a monatomic gas. Thus, this degree of freedom does not act to store heat, and does not contribute to the heat capacity of nitrogen. . . .

At higher temperatures, however, nitrogen gas gains two more degrees of internal freedom, as the molecule is excited into higher vibrational modes that store thermal energy. Now the bond is contributing heat capacity, and is contributing more than if the atoms were not bonded. With full thermal excitation of bond vibration, the heat capacity per volume, or per mole of gas molecules approaches seven-thirds that of monatomic gases. … See thermodynamic temperature for more information on translational motions, kinetic (heat) energy, and their relationship to temperature.

However, even at these large temperatures where gaseous nitrogen is able to store 7/6ths of the energy per atom of a monatomic gas (making it more efficient at storing energy on an atomic basis), it still only stores 7/12 ths of the maximal per-atom heat capacity of a solid, meaning it is not nearly as efficient at storing thermal energy on an atomic basis, as solid substances can be. This is typical of gases, and results because many of the potential bonds which might be storing potential energy in gaseous nitrogen (as opposed to solid nitrogen) are lacking, because only one of the spatial dimensions for each nitrogen atom offers a bond into which potential energy can be stored without increasing the kinetic energy of the atom. In general, solids are most efficient, on an atomic basis, at storing thermal energy (that is, they have the highest per-atom or per-mole-of-atoms heat capacity).

Per mole of different units[edit] Per mole of molecules[edit] When the specific heat capacity, c, of a material is measured (lowercase c means the unit quantity is in terms of mass), different values arise because different substances have different molar masses (essentially, the weight of the individual atoms or molecules). In solids, thermal energy arises due to the number of atoms that are vibrating. "Molar" heat capacity per mole of molecules, for both gases and solids, offer figures which are arbitrarily large, since molecules may be arbitrarily large. Such heat capacities are thus not intensive quantities for this reason, since the quantity of mass being considered can be increased without limit.

Per mole of atoms[edit] Conversely, for molecular-based substances (which also absorb heat into their internal degrees of freedom), massive, complex molecules with high atomic count—like octane—can store a great deal of energy per mole and yet are quite unremarkable on a mass basis, or on a per-atom basis. This is because, in fully excited systems, heat is stored independently by each atom in a substance, not primarily by the bulk motion of molecules. . . .

Because of the connection of heat capacity to the number of atoms, some care should be taken to specify a mole-of-molecules basis vs. a mole-of-atoms basis, when comparing specific heat capacities of molecular solids and gases. Ideal gases have the same numbers of molecules per volume, so increasing molecular complexity adds heat capacity on a per-volume and per-mole-of-molecules basis, but may lower or raise heat capacity on a per-atom basis, depending on whether the temperature is sufficient to store energy as atomic vibration. . . .

This limit of 3 R per mole specific heat capacity is approached at room temperature for most solids, with significant departures at this temperature only for solids composed of the lightest atoms which are bound very strongly, such as beryllium (where the value is only of 66% of 3 R), or diamond (where it is only 24% of 3 R). These large departures are due to quantum effects which prevent full distribution of heat into all vibrational modes, when the energy difference between vibrational quantum states is very large compared to the average energy available to each atom from the ambient temperature.

For monatomic gases, the specific heat is only half of 3 R per mole, i.e. (3⁄2R per mole) due to loss of all potential energy degrees of freedom in these gases. For polyatomic gases, the heat capacity will be intermediate between these values on a per-mole-of-atoms basis, and (for heat-stable molecules) would approach the limit of 3 R per mole of atoms, for gases composed of complex molecules, and at higher temperatures at which all vibrational modes accept excitational energy. This is because very large and complex gas molecules may be thought of as relatively large blocks of solid matter which have lost only a relatively small fraction of degrees of freedom, as compared to a fully integrated solid.

For a list of heat capacities per atom-mole of various substances, in terms of R, see the last column of the table of heat capacities below.

Corollaries of these considerations for solids (volume-specific heat capacity)[edit] Since the bulk density of a solid chemical element is strongly related to its molar mass (usually about 3 R per mole, as noted above), there exists a noticeable inverse correlation between a solid’s density and its specific heat capacity on a per-mass basis. This is due to a very approximate tendency of atoms of most elements to be about the same size, and constancy of mole-specific heat capacity) result in a good correlation between the volume of any given solid chemical element and its total heat capacity. Another way of stating this, is that the volume-specific heat capacity (volumetric heat capacity) of solid elements is roughly a constant. The molar volume of solid elements is very roughly constant, and (even more reliably) so also is the molar heat capacity for most solid substances. These two factors determine the volumetric heat capacity, which as a bulk property may be striking in consistency. For example, the element uranium is a metal which has a density almost 36 times that of the metal lithium, but uranium's specific heat capacity on a volumetric basis (i.e. per given volume of metal) is only 18% larger than lithium's.

Since the volume-specific corollary of the Dulong-Petit specific heat capacity relationship requires that atoms of all elements take up (on average) the same volume in solids, there are many departures from it, with most of these due to variations in atomic size. For instance, arsenic, which is only 14.5% less dense than antimony, has nearly 59% more specific heat capacity on a mass basis. In other words; even though an ingot of arsenic is only about 17% larger than an antimony one of the same mass, it absorbs about 59% more heat for a given temperature rise. The heat capacity ratios of the two substances closely follows the ratios of their molar volumes (the ratios of numbers of atoms in the same volume of each substance); the departure from the correlation to simple volumes in this case is due to lighter arsenic atoms being significantly more closely packed than antimony atoms, instead of similar size. In other words, similar-sized atoms would cause a mole of arsenic to be 63% larger than a mole of antimony, with a correspondingly lower density, allowing its volume to more closely mirror its heat capacity behavior.

Other factors[edit] Hydrogen bonds[edit] Hydrogen-containing polar molecules like ethanol, ammonia, and water have powerful, intermolecular hydrogen bonds when in their liquid phase. These bonds provide another place where heat may be stored as potential energy of vibration, even at comparatively low temperatures. Hydrogen bonds account for the fact that liquid water stores nearly the theoretical limit of 3 R per mole of atoms, even at relatively low temperatures (i.e. near the freezing point of water).

Impurities[edit] . . .

The simple case of the monatomic gas[edit] In the case of a monatomic gas such as helium under constant volume, if it is assumed that no electronic or nuclear quantum excitations occur, each atom in the gas has only 3 degrees of freedom, all of a translational type. No energy dependence is associated with the degrees of freedom which define the position of the atoms. While, in fact, the degrees of freedom corresponding to the momenta of the atoms are quadratic, and thus contribute to the heat capacity. There are N atoms, each of which has 3 components of momentum, which leads to 3N total degrees of freedom. This gives:

C_V=\left(\frac{\partial U}{\partial T}\right)_V=\frac{3}{2}N\,k_B =\frac{3}{2}n\,R C_{V,m}=\frac{C_V}{n}=\frac{3}{2}R where

C_V is the heat capacity at constant volume of the gas C_{V,m} is the molar heat capacity at constant volume of the gas N is the total number of atoms present in the container n is the number of moles of atoms present in the container (n is the ratio of N and Avogadro’s number) R is the ideal gas constant, (8.3144621[75] J/(mol·K). R is equal to the product of Boltzmann’s constant k_B and Avogadro’s number The following table shows experimental molar constant volume heat capacity measurements taken for each noble monatomic gas (at 1 atm and 25 °C):

Monatomic gas CV, m (J/(mol·K)) CV, m/R He 12.5 1.50 Ne 12.5 1.50 Ar 12.5 1.50 Kr 12.5 1.50 Xe 12.5 1.50 It is apparent from the table that the experimental heat capacities of the monatomic noble gases agrees with this simple application of statistical mechanics to a very high degree.

The molar heat capacity of a monatomic gas at constant pressure is then

C_{p,m}=C_{V,m} + R=\frac{5}{2}R Diatomic gas[edit]

Constant volume specific heat capacity of a diatomic gas (idealised). As temperature increases, heat capacity goes from 3/2 R (translation contribution only), to 5/2 R (translation plus rotation), finally to a maximum of 7/2 R (translation + rotation + vibration) In the somewhat more complex case of an ideal gas of diatomic molecules, the presence of internal degrees of freedom are apparent. In addition to the three translational degrees of freedom, there are rotational and vibrational degrees of freedom. In general, the number of degrees of freedom, f, in a molecule with na atoms is 3na:

f=3n_a \, Mathematically, there are a total of three rotational degrees of freedom, one corresponding to rotation about each of the axes of three-dimensional space. However, in practice only the existence of two degrees of rotational freedom for linear molecules will be considered. This approximation is valid because the moment of inertia about the internuclear axis is vanishingly small with respect to other moments of inertia in the molecule (this is due to the very small rotational moments of single atoms, due to the concentration of almost all their mass at their centers; compare also the extremely small radii of the atomic nuclei compared to the distance between them in a diatomic molecule). Quantum mechanically, it can be shown that the interval between successive rotational energy eigenstates is inversely proportional to the moment of inertia about that axis. Because the moment of inertia about the internuclear axis is vanishingly small relative to the other two rotational axes, the energy spacing can be considered so high that no excitations of the rotational state can occur unless the temperature is extremely high. It is easy to calculate the expected number of vibrational degrees of freedom (or vibrational modes). There are three degrees of translational freedom, and two degrees of rotational freedom, therefore

f_\mathrm{vib}=f-f_\mathrm{trans}-f_\mathrm{rot}=6-3-2=1 \, Each rotational and translational degree of freedom will contribute R/2 in the total molar heat capacity of the gas. Each vibrational mode will contribute R to the total molar heat capacity, however. This is because for each vibrational mode, there is a potential and kinetic energy component. Both the potential and kinetic components will contribute R/2 to the total molar heat capacity of the gas. Therefore, a diatomic molecule would be expected to have a molar constant-volume heat capacity of

C_{V,m}=\frac{3R}{2}+R+R=\frac{7R}{2}=3.5 R where the terms originate from the translational, rotational, and vibrational degrees of freedom, respectively.

Constant volume specific heat capacity of diatomic gases (real gases) between about 200 K and 2000 K. This temperature range is not large enough to include both quantum transitions in all gases. Instead, at 200 K, all but hydrogen are fully rotationally excited, so all have at least 5/2 R heat capacity. (Hydrogen is already below 5/2, but it will require cryogenic conditions for even H2 to fall to 3/2 R). Further, only the heavier gases fully reach 7/2 R at the highest temperature, due to the relatively small vibrational energy spacing of these molecules. HCl and H2 begin to make the transition above 500 K, but have not achieved it by 1000 K, since their vibrational energy-level spacing is too wide to fully participate in heat capacity, even at this temperature. The following is a table of some molar constant-volume heat capacities of various diatomic gases at standard temperature (25 °C = 298 K)

Diatomic gas CV, m (J/(mol·K)) CV, m / R H2 20.18 2.427 CO 20.2 2.43 N2 19.9 2.39 Cl2 24.1 3.06 Br2 (vapour) 28.2 3.39 From the above table, clearly there is a problem with the above theory. All of the diatomics examined have heat capacities that are lower than those predicted by the equipartition theorem, except Br2. However, as the atoms composing the molecules become heavier, the heat capacities move closer to their expected values. One of the reasons for this phenomenon is the quantization of vibrational, and to a lesser extent, rotational states. In fact, if it is assumed that the molecules remain in their lowest energy vibrational state because the inter-level energy spacings for vibration-energies are large, the predicted molar constant volume heat capacity for a diatomic molecule becomes just that from the contributions of translation and rotation:

C_{V,m}=\frac{3R}{2}+R=\frac{5R}{2}=2.5R which is a fairly close approximation of the heat capacities of the lighter molecules in the above table. If the quantum harmonic oscillator approximation is made, it turns out that the quantum vibrational energy level spacings are actually inversely proportional to the square root of the reduced mass of the atoms composing the diatomic molecule. Therefore, in the case of the heavier diatomic molecules such as chlorine or bromine, the quantum vibrational energy level spacings become finer, which allows more excitations into higher vibrational levels at lower temperatures. This limit for storing heat capacity in vibrational modes, as discussed above, becomes 7R/2 = 3.5 R per mole of gas molecules, which is fairly consistent with the measured value for Br2 at room temperature. As temperatures rise, all diatomic gases approach this value.

General gas phase[edit] Question book-new.svg This section does not cite any references or sources. Please help improve this section by adding citations to reliable sources. Unsourced material may be challenged and removed.

The specific heat of the gas is best conceptualized in terms of the degrees of freedom of an individual molecule. The different degrees of freedom correspond to the different ways in which the molecule may store energy. The molecule may store energy in its translational motion according to the formula:

E=\frac{1}{2}\,m\left(v_x^2+v_y^2+v_z^2\right) where m is the mass of the molecule and [v_x,v_y,v_z] is velocity of the center of mass of the molecule. Each direction of motion constitutes a degree of freedom, so that there are three translational degrees of freedom.

In addition, a molecule may have rotational motion. The kinetic energy of rotational motion is generally expressed as

E=\frac{1}{2}\,\left(I_1\omega_1^2+I_2\omega_2^2+I_3\omega_3^2\right) where I is the moment of inertia tensor of the molecule, and [\omega_1,\omega_2,\omega_3] is the angular velocity pseudo-vector (in a coordinate system aligned with the principal axes of the molecule). In general, then, there will be three additional degrees of freedom corresponding to the rotational motion of the molecule, (For linear molecules one of the inertia tensor terms vanishes and there are only two rotational degrees of freedom). The degrees of freedom corresponding to translations and rotations are called the rigid degrees of freedom, since they do not involve any deformation of the molecule.

The motions of the atoms in a molecule which are not part of its gross translational motion or rotation may be classified as vibrational motions. It can be shown that if there are n atoms in the molecule, there will be as many as v = 3n-3-n_r vibrational degrees of freedom, where n_r is the number of rotational degrees of freedom. A vibrational degree of freedom corresponds to a specific way in which all the atoms of a molecule can vibrate. The actual number of possible vibrations may be less than this maximal one, due to various symmetries.

For example, triatomic nitrous oxide N2O will have only 2 degrees of rotational freedom (since it is a linear molecule) and contains n=3 atoms: thus the number of possible vibrational degrees of freedom will be v = (3*3)-3-2 = 4. There are four ways or "modes" in which the three atoms can vibrate, corresponding to 1) A mode in which an atom at each end of the molecule moves away from, or towards, the center atom at the same time, 2) a mode in which either end atom moves asynchronously with regard to the other two, and 3) and 4) two modes in which the molecule bends out of line, from the center, in the two possible planar directions that are orthogonal to its axis. Each vibrational degree of freedom confers TWO total degrees of freedom, since vibrational energy mode partitions into 1 kinetic and 1 potential mode. This would give nitrous oxide 3 translational, 2 rotational, and 4 vibrational modes (but these last giving 8 vibrational degrees of freedom), for storing energy. This is a total of f = 3+2+8 = 13 total energy-storing degrees of freedom, for N2O.

For a bent molecule like water H2O, a similar calculation gives 9-3-3 = 3 modes of vibration, and 3 (translational) + 3 (rotational) + 6 (vibrational) = 12 degrees of freedom.

The storage of energy into degrees of freedom[edit] If the molecule could be entirely described using classical mechanics, then the theorem of equipartition of energy could be used to predict that each degree of freedom would have an average energy in the amount of (1/2)kT where k is Boltzmann’s constant and T is the temperature. Our calculation of the constant-volume heat capacity would be straightforward. Each molecule would be holding, on average, an energy of (f/2)kT where f is the total number of degrees of freedom in the molecule. Note that Nk = R if N is Avogadro's number, which is the case in considering the heat capacity of a mole of molecules. Thus, the total internal energy of the gas would be (f/2)NkT where N is the total number of molecules. The heat capacity (at constant volume) would then be a constant (f/2)Nk the mole-specific heat capacity would be (f/2)R the molecule-specific heat capacity would be (f/2)k and the dimensionless heat capacity would be just f/2. Here again, each vibrational degree of freedom contributes 2f. Thus, a mole of nitrous oxide would have a total constant-volume heat capacity (including vibration) of (13/2)R by this calculation.

In summary, the molar heat capacity (mole-specific heat capacity) of an ideal gas with f degrees of freedom is given by

C_{V,m}=\frac{f}{2} R . . .

The constant-pressure heat capacity for any gas would exceed this by an extra factor of R (see Mayer's relation, above). As example Cp would be a total of (15/2)R/mole for nitrous oxide.

The effect of quantum energy levels in storing energy in degrees of freedom[edit] The various degrees of freedom cannot generally be considered to obey classical mechanics, however. Classically, the energy residing in each degree of freedom is assumed to be continuous—it can take on any positive value, depending on the temperature. In reality, the amount of energy that may reside in a particular degree of freedom is quantized: It may only be increased and decreased in finite amounts. A good estimate of the size of this minimum amount is the energy of the first excited state of that degree of freedom above its ground state. For example, the first vibrational state of the hydrogen chloride (HCl) molecule has an energy of about 5.74 × 10−20 joule. If this amount of energy were deposited in a classical degree of freedom, it would correspond to a temperature of about 4156 K.

If the temperature of the substance is so low that the equipartition energy of (1/2)kT is much smaller than this excitation energy, then there will be little or no energy in this degree of freedom. This degree of freedom is then said to be “frozen out". As mentioned above, the temperature corresponding to the first excited vibrational state of HCl is about 4156 K. For temperatures well below this value, the vibrational degrees of freedom of the HCl molecule will be frozen out. They will contain little energy and will not contribute to the thermal energy or the heat capacity of HCl gas.

Energy storage mode "freeze-out" temperatures[edit] Question book-new.svg This section does not cite any references or sources. Please help improve this section by adding citations to reliable sources. Unsourced material may be challenged and removed.

It can be seen that for each degree of freedom there is a critical temperature at which the degree of freedom “unfreezes” and begins to accept energy in a classical way. In the case of translational degrees of freedom, this temperature is that temperature at which the thermal wavelength of the molecules is roughly equal to the size of the container. For a container of macroscopic size (e.g. 10 cm) this temperature is extremely small and has no significance, since the gas will certainly liquify or freeze before this low temperature is reached. For any real gas translational degrees of freedom may be considered to always be classical and contain an average energy of (3/2)kT per molecule.

The rotational degrees of freedom are the next to “unfreeze". In a diatomic gas, for example, the critical temperature for this transition is usually a few tens of kelvins, although with a very light molecule such as hydrogen the rotational energy levels will be spaced so widely that rotational heat capacity may not completely "unfreeze" until considerably higher temperatures are reached. Finally, the vibrational degrees of freedom are generally the last to unfreeze. As an example, for diatomic gases, the critical temperature for the vibrational motion is usually a few thousands of kelvins, and thus for the nitrogen in our example at room temperature, no vibration modes would be excited, and the constant-volume heat capacity at room temperature is (5/2)R/mole, not (7/2)R/mole. As seen above, with some unusually heavy gases such as iodine gas I2, or bromine gas Br2, some vibrational heat capacity may be observed even at room temperatures.

It should be noted that it has been assumed that atoms have no rotational or internal degrees of freedom. This is in fact untrue. For example, atomic electrons can exist in excited states and even the atomic nucleus can have excited states as well. Each of these internal degrees of freedom are assumed to be frozen out due to their relatively high excitation energy. Nevertheless, for sufficiently high temperatures, these degrees of freedom cannot be ignored. In a few exceptional cases, such molecular electronic transitions are of sufficiently low energy that they contribute to heat capacity at room temperature, or even at cryogenic temperatures. One example of an electronic transition degree of freedom which contributes heat capacity at standard temperature is that of nitric oxide (NO), in which the single electron in an anti-bonding molecular orbital has energy transitions which contribute to the heat capacity of the gas even at room temperature.

An example of a nuclear magnetic transition degree of freedom which is of importance to heat capacity, is the transition which converts the spin isomers of hydrogen gas (H2) into each other. At room temperature, the proton spins of hydrogen gas are aligned 75% of the time, resulting in orthohydrogen when they are. Thus, some thermal energy has been stored in the degree of freedom available when parahydrogen (in which spins are anti-aligned) absorbs energy, and is converted to the higher energy ortho form. However, at the temperature of liquid hydrogen, not enough heat energy is available to produce orthohydrogen (that is, the transition energy between forms is large enough to "freeze out" at this low temperature), and thus the parahydrogen form predominates. The heat capacity of the transition is sufficient to release enough heat, as orthohydrogen converts to the lower-energy parahydrogen, to boil the hydrogen liquid to gas again, if this evolved heat is not removed with a catalyst after the gas has been cooled and condensed. This example also illustrates the fact that some modes of storage of heat may not be in constant equilibrium with each other in substances, and heat absorbed or released from such phase changes may "catch up" with temperature changes of substances, only after a certain time. In other words, the heat evolved and absorbed from the ortho-para isomeric transition contributes to the heat capacity of hydrogen on long time-scales, but not on short time-scales. These time scales may also depend on the presence of a catalyst.

Less exotic phase-changes may contribute to the heat-capacity of substances and systems, as well, as (for example) when water is converted back and forth from solid to liquid or gas form. Phase changes store heat energy entirely in breaking the bonds of the potential energy interactions between molecules of a substance. As in the case of hydrogen, it is also possible for phase changes to be hindered as the temperature drops, so that they do not catch up and become apparent, without a catalyst. For example, it is possible to supercool liquid water to below the freezing point, and not observe the heat evolved when the water changes to ice, so long as the water remains liquid. This heat appears instantly when the water freezes.

Solid phase[edit] Main articles: Einstein solid, Debye model and Kinetic theory of solids

The dimensionless heat capacity divided by three, as a function of temperature as predicted by the Debye model and by Einstein’s earlier model. The horizontal axis is the temperature divided by the Debye temperature. Note that, as expected, the dimensionless heat capacity is zero at absolute zero, and rises to a value of three as the temperature becomes much larger than the Debye temperature. The red line corresponds to the classical limit of the Dulong-Petit law For matter in a crystalline solid phase, the Dulong-Petit law, which was discovered empirically, states that the mole-specific heat capacity assumes the value 3 R. Indeed, for solid metallic chemical elements at room temperature, molar heat capacities range from about 2.8 R to 3.4 R. Large exceptions at the lower end involve solids composed of relatively low-mass, tightly bonded atoms, such as beryllium at 2.0 R, and diamond at only 0.735 R. The latter conditions create larger quantum vibrational energy-spacing, so that many vibrational modes have energies too high to be populated (and thus are "frozen out") at room temperature. At the higher end of possible heat capacities, heat capacity may exceed R by modest amounts, due to contributions from anharmonic vibrations in solids, and sometimes a modest contribution from conduction electrons in metals. These are not degrees of freedom treated in the Einstein or Debye theories.

The theoretical maximum heat capacity for multi-atomic gases at higher temperatures, as the molecules become larger, also approaches the Dulong-Petit limit of 3 R, so long as this is calculated per mole of atoms, not molecules. The reason for this behavior is that, in theory, gases with very large molecules have almost the same high-temperature heat capacity as solids, lacking only the (small) heat capacity contribution that comes from potential energy that cannot be stored between separate molecules in a gas.

The Dulong-Petit limit results from the equipartition theorem, and as such is only valid in the classical limit of a microstate continuum, which is a high temperature limit. For light and non-metallic elements, as well as most of the common molecular solids based on carbon compounds at standard ambient temperature, quantum effects may also play an important role, as they do in multi-atomic gases. These effects usually combine to give heat capacities lower than 3 R per mole of atoms in the solid, although in molecular solids, heat capacities calculated per mole of molecules in molecular solids may be more than 3 R. For example, the heat capacity of water ice at the melting point is about 4.6 R per mole of molecules, but only 1.5 R per mole of atoms. As noted, heat capacity values far lower than 3 R "per atom" (as is the case with diamond and beryllium) result from “freezing out” of possible vibration modes for light atoms at suitably low temperatures, just as happens in many low-mass-atom gases at room temperatures (where vibrational modes are all frozen out). Because of high crystal binding energies, the effects of vibrational mode freezing are observed in solids more often than liquids: for example the heat capacity of liquid water is twice that of ice at near the same temperature, and is again close to the 3 R per mole of atoms of the Dulong-Petit theoretical maximum.

Liquid phase[edit] For a more modern and precise analysis of the heat capacities of solids, especially at low temperatures, it is useful to use the idea of phonons. See Debye model. ...

Physicists have revived concepts first put forth in the 1940s to develop a new theory of the heat capacity of liquids. Created by Dmitry Bolmatov and Kostya Trachenko the new "phonon theory of liquid thermodynamics" has successfully predicted the heat capacity of 21 different liquids ranging from metals to noble and molecular liquids. The researchers say that the theory covers both the classical and quantum regimes and agrees with experiment over a wide range of temperatures and pressures.

While physicists have a good theoretical understanding of the heat capacity of both solids and gases, a general theory of the heat capacity of liquids has always remained elusive. Apart from being an awkward hole in our knowledge of condensed-matter physics, heat capacity – the amount of heat needed to change a substance's temperature by a certain amount – is a technologically relevant quantity that it would be nice to be able to predict. Physicists had been reluctant to develop a theory because the relevant interactions in a liquid are both strong and specific to that liquid, which, it was felt, would make it tricky to develop a general way of calculating heat capacity for liquids.

Using phonons – quantized lattice vibrations that behave like particles – to develop a theory of specific heat is nothing new in the world of solids. After all, the atoms in a solid oscillate about fixed points in the lattice, which means that the only way that heat – in the form of randomly vibrating atoms – can move through a material is via phonons. Indeed, Albert Einstein and Peter Debye famously developed separate theories early in the 20th century to explain the high-temperature and low-temperature heat capacity of solids, respectively. But, given that the atoms in a liquid are free to move and so can absorb or transfer heat without any need for phonons, it is not at first glance obvious why phonons should be a good way of describing how heat is transferred and absorbed in a liquid. Anyone who has dunked their head under water knows that sound propagates very well in liquids – in the form of longitudinal phonons. What is not obvious, though, is whether transverse or "shear" phonons, which exist in solids, also occur in liquids. Because each phonon mode contributes to the specific heat, it is very important to know how many modes occur in a liquid of interest.

This problem was first tackled in the 1940s by the Russian physicist Yakov Frenkel. He pointed out that for vibrations above a certain frequency (the Frenkel frequency), molecules in a liquid behave like those in a solid – and can therefore support shear phonons. His idea was that it takes a characteristic amount of time for an atom or molecule to move from one equilibrium position in the liquid to another. As long as the period of the vibration is shorter than this time, the molecules will vibrate as if they are fixed in a solid.

With this in mind, Bolmatov and colleagues derived an expression for the energy of a liquid in terms of its temperature and three parameters – the liquid's coefficient of expansion, and its Debye and Frenkel frequencies. The Debye frequency is the theoretical maximum frequency that atoms or molecules in the liquid can oscillate at and can be derived from the speed of sound in the liquid. The Frenkel frequency puts a lower bound on the oscillation frequency of the atoms or molecules and can be derived from the viscosity and shear modulus of the liquid.

The result is an expression for specific heat as a function of temperature that can be compared with experimental data. In all 21 liquids studied, the theory was able to reproduce the observed drop in heat capacity as temperature increases. The physicists explain this drop in terms of an increase in the Frenkel frequency as a function of temperature. As the material gets hotter, there are fewer shear phonon modes available to transport heat and therefore the heat capacity drops.

The theory was able to describe simple liquids – such as the noble liquids, which comprise atoms – through to complicated molecular liquids such as hydrogen sulphide, methane and water. The physicists say that this broad agreement suggests that Frenkel's original proposal that the phonon states of the liquid depend upon a characteristic time applies to a wide range of materials. The result is that physicists should be able to predict the specific heat of many liquids without having to worry about complicated interactions between constituent atoms or molecules.

Bolmatov told Physics World that there are two reasons why it took so long for Frenkel's ideas to be applied to heat capacity. "The first is that it took 50 years to verify Frenkel's prediction," he says. The second reason is that historically the thermodynamic theory of liquids was developed from the theory of gases, not the theory of solids – despite the similarities between liquids and solids. "This development had a certain inertia associated with it and consequently resulted in some delays and more thought was required for proposing that Frenkel's idea can be translated into a consistent phonon theory of liquids."[citation needed]

. . .

Table of specific heat capacities[edit]

This section needs additional citations for verification. Please help improve this article by adding citations to reliable sources. Unsourced material may be challenged and removed.

See also: List of thermal conductivities Note that the especially high molar values, as for paraffin, gasoline, water and ammonia, result from calculating specific heats in terms of moles of molecules. If specific heat is expressed per mole of atoms for these substances, none of the constant-volume values exceed, to any large extent, the theoretical Dulong-Petit limit of 25 J·mol−1·K−1 = 3 R per mole of atoms (see the last column of this table). Paraffin, for example, has very large molecules and thus a high heat capacity per mole, but as a substance it does not have remarkable heat capacity in terms of volume, mass, or atom-mol (which is just 1.41 R per mole of atoms, or less than half of most solids, in terms of heat capacity per atom).

In the last column, major departures of solids at standard temperatures from the Dulong-Petit law value of 3 R, are usually due to low atomic weight plus high bond strength (as in diamond) causing some vibration modes to have too much energy to be available to store thermal energy at the measured temperature. For gases, departure from 3 R per mole of atoms in this table is generally due to two factors: (1) failure of the higher quantum-energy-spaced vibration modes in gas molecules to be excited at room temperature, and (2) loss of potential energy degree of freedom for small gas molecules, simply because most of their atoms are not bonded maximally in space to other atoms, as happens in many solids.

The shear amount of this unsourced material is staggering.

The attitude of the persons contributing these large volumes of unverifiable material appears to be "just trust us"—I have never seen such an essay-quality evolution of a chemistry article, to the extent that it has developed here. Needless to say, it flies in the face of WP policies to let it remain this way for much longer. And it is entirely impractical to expect any editor to analyze the text and draw together sources to forensically add citations post hoc, to remediate the sloppy scholarship originally presented. This leaves taking out block after block, and replacing it with real encyclopedic writing that is tied to the views of published works. I challenge those committed to this article to do something about it. Were this a student contribution to me, I would take scissors to it in their presence, handing back to them the sourced information (i.e., sending to the floor what appears here, and returning too them as an adequate start the small portions omitted from above that HAVE references). I will not do the same here, but challenge you, in the name of WP polcieis and good encyclopedic writing, to do such a thing over time.

FInally, note, while I have not argued it (as being the more esoteric of the ways to approach this issue), it must be clear to any contributor with proper training that the more one knows about this subject, the more necessary and motivated contributors should be to cite their sources (given the rich history of this field, the various schools of thought that exist regarding its theories, the various philosophies that exist for the presentation of its ideas, etc.). In my graduate course related to this, taught by 2 of the 3 of the BRR PChem trio, [3], every effort was made to tie ideas to their sources. The writers here are no better prepared for presenting this, and certainly, given the context and WP policies, are in no better position to demand de novo trust in their unsourced essaying. 71.239.87.100 (talk) 00:51, 15 February 2015 (UTC)[reply]

Second major issue, based on scrutiny of sourcing that does appear[edit]

The second issue is that for the parts of the text that are sourced, the article is poorly referenced, on several levels (in re: WP policies and guidelines). Hence, the tags that currently appear will be joined by one further, based on the following survey of the sourcing that does appear in this article.

For the 30 page, ≈80 paragraph, 10,000 word article, there are 42 inline citations, to 34 sources. Apart from one undergraduate text cited three times, and a web data source for tabulated data cite 8 times, all sources are cited once. Hence, if not for the long unsourced tracts discussed above, this trove might seem to suggest the beginning of a good article. However, there are issues small and large that defy this conclusion.

The simple issues are important, but are of smaller magnitude here; there is only one pair of repeated citations [citns. 7,8], three bare URLs [21, 28, 29], and one dead link [citn. 18]. And there are issues with completeness of citations, and some are significant one—14 of the 34 citations missing at least some of the standard information required by good referencing practice [3, 4, 5, 7, 14, 18, 19, 21, 23, 28, 29, 30, 31, 32] including 9 references missing page numbers, either completely lacking or having undefined page ranges [2, 4, 7, 8, 9, 17, 22, 23, 26, mostly all lacking].

Rather, after the foregoing major issue of large tracts of unsourced information, the issue with the remainder (which is sourced, at east to a degree) is that it fails basic tests of being derived from reliable secondary science sources, so that OR is not necessary to compose ones arguments.

Of the sources that are not being used to derive tabulated data (see below), the remaining, principal citations of the article breakdown as follows. There are:

  • 7 undergraduate texts, referenced 9 times [1, 7, 8, 14, 15, 18, 23]
  • 2 advanced texts, cited twice [17, 33]
  • 1 review article, cited once [26]
  • 11 references to primary sources [9, 10, 11, 12, 13, 20, 22, 24, 25, 34, 35]
  • 6 problematic citations of web-only sources (non-institutional, unrefereed sources), including one which is an unattributed book chapter [6, 19, 21, 28, 29, 32]

In addition to these, there are two citations that are primarily technical about nomenclature and the like [3, 4], and three citations of primarily historical/philosophical works [2, 13, 20].

A look at these numbers makes clear that the small part of the article that has citations, draws them not from the mandated advanced secondary sources—reviews, and advanced monographs and texts (here, only ~10%)—but rather from undergraduate textbooks (about 1/3 of inline citations) or primary sources. The details of these latter two categories of sources further highlights the issues with these. A full eight (almost all of) the inline citations to the textbooks are to books that appear without page numbers for the material purportedly sourced, including 3 refs to same text. In the cases of primary sources use, they are not used as a small part of the sourcing further covered by secondary sources (the approved manner of using primary sources); instead the primary sources appear as the sole source of the information in their respective sections: the "Negative specific heat" section, all of citations [10-13] are primary, and in the 940 word "Liquid phase" section, 3 citations appear for but 2 of the 10 paragraphs, and these three are all primary [24, 25. 26]. (Curiously, these primary sources are imbued with greater import and future problems, in being overly out-linked to abstracts (arXic, DOI, Bibcode, PMID, etc.), sometimes 3 times in the same bibliographic entry, a nightmare for future link maintenance.)

The bottom line of this analysis is that in addition to the vast portions of clearly unsourced text, much of the apparently sourced text is effectively unverifiable (linked only to texts without page numbers, or to poor web sources), and of the remainder, most of it is non-representative specialist information (astronomy interests, negative heat capacity content, some liquid phase content) that is supported only by primary sources, sometimes by repeat references to the same lab. In short, even the small part of the article for which references appear would not receive a passing mark for the quality of its sourced writing.

Finally, and supportive of what appears to be a rampant degree of essay type writing, and WP:OR, I would note that about 20% of the citations are to raw data sources, in support of tables or arguments. Of these only three are to published data sources [4, 5, 34]; the remainder are to web-only data sources [28, 29, 30, 31]. The oddity of this goes further; rather that pulling Tipler, the writers make 7 references to published Tipler data, citing only the web reiteration of the data (which may have been misrepresented in its selection, may have been mistranscribed from the published sources, etc.), this even though a direct link is given to the Tipler source at the cited, derivative web-source.

Bottom line, all of this reinforces the conclusion that this article is fundamentally flawed, and so needs a major shift in approach, moving away from unsourced and poorly sourced content, to the best content, created from the best available sources. And there is no shortage of great secondary sources, in this field.

I will try to add to the Further reading section, as help to redirecting this material toward the best available material. 71.239.87.100 (talk) 03:11, 15 February 2015 (UTC)[reply]

You are correct. This article is proof that the less a person knows about a subject the more they drag on and on about it. I am surprised you spent so much time going through this morass of "information." You should suggest to some of your graduate students to take this as an opportunity to gain some writing experience and give this article a thorough work over. Editing these articles can be fun. I suspect when it is properly written it will have shrunk to one tenth its present size.Zedshort (talk) 02:55, 4 March 2016 (UTC)[reply]

Liquids[edit]

I'm adding this comment here because I'm not sure where it goes. I'm a professional physicist, and my opinion of the section on Liquids is that it has been added as an attempt at self-promotion. Ref 24 (an article in a fringe journal Scientific Reports and a writeup in Physics World) by Bolmatov et al, hardly establishes this work as fundamental to the field. There is a very long discussion of it and it really reeks of self-promotion. Can we agree to remove it? — Preceding unsigned comment added by 129.67.66.148 (talk) 09:28, 22 May 2015 (UTC)[reply]

I'm not qualified to judge on its removal, but 129.67.66.148 makes a good case. Even if the material does not deserve the label of "self-promotion" the section is much longer than necessary. Spiel496 (talk) 23:03, 23 May 2015 (UTC)[reply]
Professional physicist here, as well. I don't think Scientific Reports can be considered a fringe journal, but this section is at least some type of promotion, and the section is longer than needed. Given the discussion here, I will trim it down significantly. It seems that this Dmitry Bolmatov has also been added to the List of Russian physicists, despite not being as famous as the other Russian physicists listed. There might be some promotion going on here. Mgibby5 (talk) 21:01, 11 July 2015 (UTC)[reply]
I typed some of the words from the section into google and got out that this text quotes directly from this article by physicsworld for almost all of its text. It seems that this is a pretty clear-cut case of plagiarism. I am going to be bold and remove the section entirely, to be replaced with something short and more generic. — Preceding unsigned comment added by Mgibby5 (talk • contribs) 21:16, 11 July 2015 (UTC)[reply]

mass-specific heat is also an assumed convention in engineering applications, not in fundamental physics[edit]

These previous objections to the overly glib pronouncements in the article's intro, and here in the discussion, on the meaning of "specific" with regard to specific heat (specifically) are indeed warranted. One educated in the either the history of development in thermodynamics, or aware of how statistical thermodynamics is utilized in current models, is aware of the fact that the heat capacity is normally required to be something other than mass-specific (e.g., Planck, Einstein, etc., effectively utulize a volume-specific heat capacity, since molar-specific at constant volume - Cv - is both volume-specific and molar-specific).

More to the point, a mass-specific heat capacity is not one iota more "physical" than a volume-specific heat capacity, since both are equally insufficient as a fundamental thermodynamic variable Pretending otherwise, as in this article, is misleading. Wikibearwithme (talk) 18:07, 1 January 2016 (UTC)[reply]

External links modified[edit]

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Magnetocaloric effect[edit]

Magnetocaloric effect is a nice way of showing how degrees of freedom and temperature relate; (gadolinium in and out of strong magnetic fields). see: https://en.wikipedia.org/wiki/Magnetic_refrigeration maybe add it as a link or use as example. 71.139.161.30 (talk) 23:43, 3 December 2016 (UTC)[reply]

Degenerate matter[edit]

What is the heat capacity of degenerate matter? What should be entered into an equation to model the cooling of a white dwarf, for instance?

2600:1700:4CA1:3C80:C8D2:18ED:2941:6B2D (talk) 01:05, 6 August 2018 (UTC)[reply]

Time to split "specific heat" off from this article[edit]

I tried to clean up this article, but it is still way too long and jumbled. It is not serving its goal -- informing the typical reader who needs to know about the topics covered.
The biggest problem is the attempt to cover "heat capacity" (extensive) and "specific heat" (intensive) in the same article. It would be like having a single article for "mass" and "density", for "distance" and "speed", for "planet" and "mineral"...
While related, they are quite distinct concepts, and deserve their own separate articles. There is very little that would need to be shared by both articles. From examples to theoretical "ab initio" computations, most of the material is specific to only one of those two topics. (And some material, like the units of measurement of heat, should be moved to the heat article instead.)
Moreover, other articles that need to link to "specific heat" now must link to a section of this one (or maybe two or three widely separate sections!), which probably do not give the reader the desired information...
Unless there are good arguments to the contrary, I intend to do the split next week, myself.
All the best, --Jorge Stolfi (talk) 06:47, 12 April 2019 (UTC)[reply]

I disagree. The article can be shortened in other ways, nobody needs, nor reads, all the formalism. Algebraic exercises don't belong in a general purpose encyclopedia. Kbrose (talk) 22:14, 8 May 2019 (UTC)[reply]
I fully agree that algebraic derivations and proofs should not be given (unless the derivation itself is the topic of the article, which is not the case here). I will try to do that cleanup too.
But that is a separate issue.
I noticed that the articles were once separate, but were merged without sufficient discussion and without a valid justification. That was a mistake, because there are four clearly distinct concepts here. Since Wikipedia is meant to be a reference work, not a textbook, distinct concepts should be put in separate articles. That will be better for readers who are looking for information in one of those concepts, and better for editors who have to link to it. --Jorge Stolfi (talk) 23:49, 10 May 2019 (UTC)[reply]
That is not convincing, and the topics are not distinct. Please seek wider consensus first. Kbrose (talk) 02:25, 11 May 2019 (UTC)[reply]
The article ias a mess, as you say, and has not been improved in TWO YEARS. The split request has been posted for one month. You have been the only one objecting to the split, with no real arguments (in fact, you only commented about the derivations, not the split itself).
If you don't have the time to improve the article, at least do not prevent others from doin so.--Jorge Stolfi (talk) 03:24, 11 May 2019 (UTC)[reply]
Don't put words into my mouth. Splitting the article is not improving it. There is no didactic reason to split the topic, and it just causes a lot of duplication of similar reasoning and formalism. If ever, the article was apparently at least more stable in the combined form than previously. At least one article, this article, should provide the comprehensive overview of the subject matter. Kbrose (talk) 12:24, 11 May 2019 (UTC)[reply]
The "didactic" reasons to split the article are obvious, in fact. A reader who needs to know what is specific heat should not have to wade though a discussion of molar heat capacity or volumetric heat capacity, and vice-versa. There may *seem* to be duplication, because the topics are *similar* -- but quite different in crucial details. (And "didactic" is not quite the right word. Wikipedia is meant to be a reference source, not a textbook.).
I am still not done; the mess was huge. Please wait a few more days beore deciding whether it was an improvement or not. --Jorge Stolfi (talk) 03:59, 14 May 2019 (UTC)[reply]
I fail to see how dividing by mass/moles/volume is really worthly of a different page. If that was true, then we would need a page in CGS units, and one in metric units, and one in fairy dust units.... AManWithNoPlan (talk) 22:07, 13 May 2019 (UTC)[reply]
Mass, moles, and volume are not just different units! They are different physical quantities. Specific heat and molar heat capacity are like people per square mile and population per country. Molar heat capacity is not even defined for many substances, whereas specific heat is. --Jorge Stolfi (talk) 03:59, 14 May 2019 (UTC)[reply]
Perhaps we need separate simple pages for each, and then a single huge "theory of heat capacity" page that they all link to if you need to understand energy levels, etc. AManWithNoPlan (talk) 15:46, 14 May 2019 (UTC)[reply]
That apparently was the idea of the unsplit article. However there does not seem to be such a thing as a "theory of heat capacity". Instead there are:
  1. Some general properties of heat capacity of general objects such as additivity. However, the heat capacity cannot even be defined for general thermodynamic systems, and the C_P x C_V discussion is very limited. (Consider a gas enclosed in a metal can with tops that can bulge out under pressure.)
  2. A theory for the specific heat of relatively homogeneous substances where one can define c_P and c_V and relate them to the equation of state and the internal energy function of the material. This theory can be applied to any homogeneous gases, liquids, or solids, including solutions, substances with unknown molecular size (like polymers) and aggregaes that are homogeneous only at a macroscopic scale, such as milk, clay, concrete, sand, etc.. It could even be extended to non-isotropic materials.
  3. A theory for the molar heat capacity of subtances with well-defined composition, homogeneous at the molecular scale based on the notion of molecular/atomic degrees of freedom. This theory can be applied to gases and (not very dense) mixtuers thereof, and (separately) to solids that are homogeneous enough for the density of DoFs to be defined.
While the last two theories are connected, their exposition is largely independent. Thus they fit well in the respective articles. (Theory 2 is currently in heat capacity but I intend to move it right away to specific heat capacity since both apply to the same class of materials.)
All the best, --Jorge Stolfi (talk) 06:13, 15 May 2019 (UTC)[reply]
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