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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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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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.
Agnius Vasiliauskas
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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.
