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When writing a chemical reaction, one can use subscripts to denote the phase of a reagent, e.g. $\ce{SiO2 (s)}$, $\ce{H2O (l)}$, $\ce{F2 (g)}$ or $\ce{Na^+ (aq)}$. However, I need to write a reaction involving a reagent that is above its critical point. What is the subscript for a supercritical reagent?

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  • $\begingroup$ I don't think there are simple subscripts for more exotic phases. $\endgroup$ – Mithoron Oct 23 '15 at 16:25
  • $\begingroup$ @Mithoron How should I notate it then? $\endgroup$ – AJMansfield Oct 23 '15 at 16:27
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    $\begingroup$ Just a note on the notation itself: the phases should not be set as subscripts. It is still very common, but the recommendations are now to put it in line, I.e. $\ce{H2O(l)}$. $\endgroup$ – Martin - マーチン Oct 24 '15 at 10:37
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The accepted if not standardised notation is Sc or sc - I've seen both. Capital "S" followed by a lower case "c", e. g. ScCO2 is the one I've seen most often in chemistry related papers. EDIT - apologies, this is a prefix and not a subscript. Hopefully you get the point

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I think the proper answer is (g), the same as a gas. After all, the critical point of hydrogen is -240 °C and 2.24 atm. But no one refers to a compressed cylinder of hydrogen gas as "supercritical hydrogen". It's just a compressed gas.

After all, supercritical fluids meet the formal definition of a gas: they have neither a defined shape nor a defined volume. (Liquids have no defined shape but do have a defined volume.) The critical transition is the loss of stability of the condensed (liquid) phase; in contrast, both before and after the critical point there is a a well-defined gas phase.

Usually when people say "supercritical" they mean "above the critical point, but not by much" or something similar. It's a not-well-defined term. I'd avoid using a special abbreviation for it, since supercritical fluids really are just gases.

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