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I do a lab where we mix a solution of sodium carbonate with a solution of calcium nitrate. As expected, calcium carbonate (white solid) precipitates out. However, if the concentration is high enough, when the solutions are first mixed a translucent, goopy substance forms. Continuing to stir that substance for another minute or two always produces the white solid.

The goopy substance does not form if the solutions are dilute enough (~2 g or less of CaCO3 forming in ~100 mL of water) but always forms if the solutions are more concentrated (~4 g or more of CaCO3 forming in ~100 mL of water).

I'm trying to figure out what that goopy substance is.

Also, it only forms once the two solutions are mixed. (So it's not as if it's just the calcium nitrate being hygroscopic). My best guess is it's some form of hydrated calcium carbonate, where the ions haven't settled into their most stable crystal lattice set and still have water molecules trapped between them, but I'm not sure.

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    $\begingroup$ Have a look at an open access article, "Pre-nucleation clusters as solute precursors in crystallisation" it discusses calcium carbonate system in detail. See the link: pubs.rsc.org/en/content/articlehtml/2014/cs/c3cs60451a. Your hypothesis seems plausible but proving it can be a nightmare because it will require too many advanced techniques. $\endgroup$
    – ACR
    Commented Mar 1, 2023 at 1:06
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    $\begingroup$ A better scientific replacement for the word "goopy" is gelatinous precipitate. $\endgroup$
    – ACR
    Commented Mar 1, 2023 at 1:07

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