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Water at absolute zero is, I suppose, ice. At room temperature it's water. At a certain point steam. What happens to it as we approach infinite temperature? (what we might call "absolute" hot)

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    $\begingroup$ Even at a balmy 2000 degrees Celsius you begin to thermally break down the water molecules so you needn't even consider "infinite" temperature $\endgroup$
    – Triatticus
    Commented Jun 16, 2023 at 11:03
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    $\begingroup$ Side note: "absolute hot" would perhaps better describe negative zero. $\endgroup$
    – Sten
    Commented Jun 16, 2023 at 20:43
  • $\begingroup$ Do you recognise any difference between 'absolute' and 'infinite'? Your ideas might be wholly valid but if that's true I, for one, suggest they suffer from your explanation. What is your 'absolute hot', please? How could there be any 'absolute hot', unless that involved both all energy and all matter? Who doubts that 'absolute zero' is the temperature at which no movement or interaction is possible? Doesn't what happens to water as not 'we' but 'it' approaches 'infinite' temperature depend mostly on your definition of 'infinite temperature'? $\endgroup$ Commented Jun 16, 2023 at 21:56
  • $\begingroup$ Related posts about "absolute hot" if you are interested: physics.stackexchange.com/questions/109738/… .. physics.stackexchange.com/questions/1775/… $\endgroup$ Commented Jun 17, 2023 at 4:44
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    $\begingroup$ I think the guy just means "very, very hot". You don't need to jump straight to infinities, just explain the interesting things that happen as matter gets hotter. $\endgroup$
    – Greenaum
    Commented Jun 17, 2023 at 7:41

3 Answers 3

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As temperature rises, you break more and more bonds between particles.

  • Going from solid to liquid, you break a number of weak chemical bonds, leaving only strong intermolecular ones like hydrogen bonds.
  • Going from liquid to gas, you break the last remaining intermolecular bonds.
  • The next to go are the electrostatic bonds between electrons and nucleus, so you get different sorts of plasmas. (Plasma formation also breaks the intramolecular chemical bonds between the atoms.)
  • After that, you start to break strong bonds between protons and neutrons and you get a thermonuclear plasma.
  • The last to go are the bonds between quarks, and you get a QGP (quark-gluon plasma). Within the known laws of physics, that's as far as you can go.
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    $\begingroup$ "Going from liquid to gas, you break the last remaining molecular bonds." What are you talking about? Most gases I know of are molecular, so surely this is not true. $\endgroup$
    – jkej
    Commented Jun 16, 2023 at 20:23
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    $\begingroup$ *inter molecular I assume, and we've skipped a step where molecules break apart $\endgroup$
    – CharlieB
    Commented Jun 16, 2023 at 20:54
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    $\begingroup$ Even higher, you probably trigger the cosmic inflation. $\endgroup$
    – fraxinus
    Commented Jun 16, 2023 at 21:11
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    $\begingroup$ @jkej Yes, I meant intermolecular bonds. As for intramolecular bonds, the point at which they break is rather complex and I hid it all behind "all sorts of plasmas". $\endgroup$
    – Miyase
    Commented Jun 16, 2023 at 23:47
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    $\begingroup$ This is a very nice qualitative breakdown. Would be even better if you could provide approximate temperatures for each stage. $\endgroup$
    – Seb
    Commented Jun 17, 2023 at 10:47
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What happens to it as we approach infinite temperature ?

Planck temperature is about $T_P = 10^{32}~\text{K}$. Wiki quotes that at this temperature :

Hypothetically, a system in thermal equilibrium at the Planck temperature might contain Planck-scale black holes, constantly being formed from thermal radiation and decaying via Hawking evaporation. Adding energy to such a system might decrease its temperature by creating larger black holes, whose Hawking temperature is lower.

Let me be lame and call this process a "quantum vacuum boiling", because micro-black holes constantly comes into existence for a very short times and evaporates instantly. Some analogies to an air bubbles forming in a boiling water can be seen.

Unfortunately,

There are no known physical models able to describe temperatures greater than $T_P$

So not much use going beyond Planck temperatures.

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    $\begingroup$ For those who wonder about the relation between this answer and Miyase's accepted answer: The Planck temperature is way, way above the temperatures considered by Miyase. Also note the word "Hypothetically", this is basically speculation since we have no way of testing these conditions. $\endgroup$ Commented Jun 19, 2023 at 10:25
  • $\begingroup$ @StigHemmer What is speculation and what's not,- depends on point of view. I prefer such position, which considers speculation a "theories" which has conjectures that are not falsifiable. As soon as we reach Planck temperatures in experiments (or notice such conditions in cosmos),- we can test this idea or maybe we can simulate numerically these extreme conditions and notice some patterns which may be tested at lower temperatures. There's no law which would forbid to reach $T_P$, hence this idea is falsifiable and so,- not a speculation. The thing that we can't do it now is irrelevant. $\endgroup$ Commented Jun 19, 2023 at 12:51
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    $\begingroup$ @AgniusVasiliauskas: You may prefer to define "speculation" that way, but in English the word means "guessing the answer without having enough information to be certain", so using it to describe theoretical predictions about an extreme situation that have never been tested is entirely appropriate. Whether it's falsifiable or not isn't relevant. $\endgroup$
    – psmears
    Commented Jun 19, 2023 at 16:07
  • $\begingroup$ @psmears Not all guessing without enough information is equal. Guessing with scientific theory based on rigorous mathematical apparatus and logic is different than guessing what you will eat at breakfast 10 days into the future. Hence "guessing" has "degrees of strength" which are completely discarded by simple English definition. $\endgroup$ Commented Jun 20, 2023 at 6:38
  • $\begingroup$ @AgniusVasiliauskas: You do you :) $\endgroup$
    – psmears
    Commented Jun 20, 2023 at 9:00
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Temperature is meaningless for a single molecule. Temperature gives you a range of possible magnitudes for the various energetic degrees of freedom, be they rotational modes, vibrational modes, translational velocity, electronic states, etc. As temperature increases, so does the average magnitude of each energy vector per molecule as the energy absorbed by the system is distributed between the different molecules.

The phase transitions from solid to liquid to gas occur as a result of kinetic energy adsorbed between each molecule (or atom) forcing them to jostle and bump until they have sufficient kinetic energy to break away from each other further and further apart. As this is happening, the movement of charge due to the kinetic energy produces electromagnetic radiation, first in the form of radio waves, then microwaves, then infrared as the phase transitions occur in the case of water. This is blackbody radiation, and as the temperature increases, the wavelengths of the radiation emitted by the matter shrink on average.

Once a certain kinetic energy is reached, electronic energy states in molecules begin to become excited both due to collisions and from absorbing blackbody radiation from another molecule. Various processes can occur from there, with the majority being fluorescence or internal conversion at lower temperatures, and then move on to bond breaking at higher temperatures. Eventually the electrons have enough energy to escape the valence orbitals of each molecule, resulting in a plasma. By this point, the water plasma is glowing in the visible range.

From there things just start breaking down further and further- the highest rated answer goes into this- and emitting shorter and shorter wavelengths, from optical to ultraviolet, to x-rays, and then to gamma-rays. The theoretical hottest anything can get is when the blackbody radiation it emits has a wavelength as short as a Planck length. By that point you would only have elementary particles moving at relativistic speeds instead of water.

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