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The title asks the entire question, but I'll elaborate:

Precipitates form in aqueous reactions when some product is insoluble in water. Is there a reaction in some other solvent where solid water forms as an insoluble product -- solid/insoluble rather than liquid/immiscible because it happens below water's freezing point?

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  • $\begingroup$ Nothing particular in mind, but guessing any reaction in cold nonpolar solvents where water is eliminated. $\endgroup$
    – Poutnik
    Oct 30, 2021 at 18:33
  • $\begingroup$ That being said, most reactions where water is a product either wouldn't go at low temperature, or would raise the temperature as they go. Finding a reaction which evades both these obstacles is somewhat of a challenge. $\endgroup$ Oct 30, 2021 at 18:54
  • $\begingroup$ One idea is to use a cryptand to dissolve sodium or potassium hydroxide and tgen add a carboxylic acid. The main obstacle with this approach is getting a solvent where this works and the freezing point is low enough. Benzene, for instance, disappointingly freezes at +5.5°C. $\endgroup$ Oct 30, 2021 at 19:22
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    $\begingroup$ Closest I can think of are the Icicles of Death: thescienceexplorer.com/nature/…. $\endgroup$
    – Ed V
    Oct 31, 2021 at 1:02
  • $\begingroup$ I have a feeling Oscar's suggestion is closest to the mark. Suitable solvents would be toluene (m.p. -95 °C) or possibly better, chloroform (m.p. -63 °C), both of which dissolve water to less than 0.1% by weight at r.t. (and even lower at sub-zero temperatures). I'd suggest tetrabutylammonium hydroxide as a base instead, but preparation of anhydrous TBAOH solutions is rather difficult, so not all of the water that "crashes out" would come from the acid-base reaction itself. $\endgroup$ Oct 31, 2021 at 8:26

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How about ammonium hydroxide, $\ce{NH4OH}$, and hydrogen chloride, $\ce{HCl}$?

At a concentration of 33% $\ce{NH3/H2O}$ by weight, the melting point (m.p.) is 173 K (-100 °C)! A 40% hydrochloric acid solution has m.p. of 247 K (-26 °C). The reaction yields $\ce{NH4Cl}$ and $\ce{H2O}$, and both would certainly be solids at those temperatures... though since ammonium chloride is exceedingly soluble, the water might be tied up as "water of hydration".

BTW, Harry Clement Stubbs, AKA Hal Clement, wrote some excellent science fiction such as Mission of Gravity and Star Light, drawing on the physics and chemistry of ammonia and water, among other things.

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    $\begingroup$ Would they, really? Soluble salts tend to keep water liquid well below 0°C. Also, the neutralization reaction is highly exothermic. $\endgroup$ Oct 31, 2021 at 6:41
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    $\begingroup$ Ammonium hydroxide do not exist. It is a solution of $\ce{NH3}$ in water $\endgroup$
    – Maurice
    Oct 31, 2021 at 22:12
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We could precipitate a predominantly water-ice phase from ... (sea)water. Methane clathrate-hydrates are commonly made on sea-floors by organic reduction of carbon dioxide[1]. One factor favoring clathrate hydrate formation is the relatively low solubility of the methane in liquid water, which enables the clathrate to form with methane while the carbon dioxide from which it is derived remains dissolved in the liquid. Other common gases such as carbon dioxide itself and hydrogen sulfide are more soluble and clathrate hydrates involving these are much rarer on Earth.

See also this answer from Earth Science SE.

Reference

  1. W. P. Dillon, in Robert A. Meyers, Encyclopedia of Physical Science and Technology, Third Edition (2001).
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