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I know there is such a thing as a supercritical fluid, where the vapor and liquid phases are indistinguishable, but is there such a thing as a supercritical state for ice? Another way of asking this is: what happens to ice when it is subjected to pressures above the critical pressure, 217.75 atm, in the case of water?

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    $\begingroup$ Contrary to most substances, increasing the pressure of solid water turns it into liquid. Then, even if you surpass the critical pressure, it would not be a supercritical fluid if you do not surpass the critical temperature. $\endgroup$ Commented Nov 28, 2023 at 6:27
  • $\begingroup$ See the water phase diagram on Physics SE $\endgroup$
    – Poutnik
    Commented Nov 28, 2023 at 8:15

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You ask three things at once, thinking they are one thing, which they are not.

  1. Does the solid/liquid line end with a critical point, like the liquid/gas line? No, it just keeps going on and on.
  2. Does anything happen to ice at water's critical pressure? Nothing at all; liquid water does not have an abrupt change of any properties at that point, neither does ice. The critical pressure is only important in relation to the liquid/gas line. For the solid/liquid line, it is about as non-special as a random point on a highway in Arizona, with tumbleweeds and Morricone music, with hundred miles behind you and hundred miles ahead.
  3. Does anything eventually happen as we increase pressure more than that? Why, yes, a lot of wonderful things happen (see the diagram below). We'll get other crystal form of ice instead of the one we were used to, and then another, and then yet another, and then more. Some of them melt at fairly high temperatures. Come to think of it, having multiple crystalline forms is not a rare thing at all. If anything, water is special in that it is better studied than most other compounds, and that's why so many forms are known.

Water phase diagram (source: Wikimedia Commons)

So it goes.

(P.S. Yes, I've read Cat's Cradle too.)

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  • $\begingroup$ That is absolutely fascinating! I will continue doing research on the myriad variations of ice. $\endgroup$
    – suse
    Commented Nov 29, 2023 at 14:25
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Water does not have a known critical point apart from the familiar one involving the usual liquid and vapor phases at about 374°C, 22 MPa.

Cerium is a different story. The phase diagram pictured below shows two fcc phases, labeled $\alpha$ and $\gamma$, that are distinct at ambient pressure but merge into a single fcc phase at roughly 250°C and 2 GPa.

Cerium phase diagram

Source

The discussion of physical characteristics given by Wikipedia implies that at low pressure the lower-temperature $\alpha$ has a higher metallic-bonding valence (from more involvement of the $4f$ orbitals in the bonding) making it denser and more conductive than $\gamma$, but this metallurgical difference decreases upon compression and thus the phases merge:

Cerium has a variable electronic structure. The energy of the 4f electron is nearly the same as that of the outer 5d and 6s electrons that are delocalized in the metallic state, and only a small amount of energy is required to change the relative occupancy of these electronic levels. This gives rise to dual valence states. For example, a volume change of about 10% occurs when cerium is subjected to high pressures or low temperatures. It appears that the valence changes from about 3 to 4 when it is cooled or compressed.1

Cited Reference

  1. Johansson, Börje; Luo, Wei; Li, Sa; Ahuja, Rajeev (17 September 2014). "Cerium; Crystal Structure and Position in The Periodic Table". Scientific Reports. 4: 6398. Bibcode:2014NatSR...4E6398J. doi:10.1038/srep06398. PMC 4165975. PMID 25227991.
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  • $\begingroup$ @Mithoron what did you do? It looks like my illustration was changed for an identical one. $\endgroup$ Commented Dec 25, 2023 at 11:22
  • $\begingroup$ tsk, tsk you didn't check the code. I only added description. $\endgroup$
    – Mithoron
    Commented Dec 25, 2023 at 12:56

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