Why does the addition of carbon dioxide to calcium hydroxide create the precipitate calcium carbonate? What happens chemically for such a molecule to form? How exactly do their interactions come into play?

Now CaOH isn’t terribly soluble in water but the parts that do dissolve, disassociate into their respective OH- and Ca2+ ions. So when we bubble CO2 through the CaOH solution the calcium ion is attracted to the partial negative charge on the oxygen atoms of carbon dioxide. So that forms CaCO2 so my question is where does the other oxygen of calcium carbonate come from? Is it snatched from a hydroxide ion?

Calcium carbonate also has a resonant structure, I don’t know what that is exactly but it’s like electrons are being shared uniformly throughout a given structure so that means carbons original double bonds get disbanded too I guess? If anybody can explain what happens here as well, it would be great.

Here are my drawings, they may be simplified but it’s all I know: enter image description here

enter image description here

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    $\begingroup$ There are millions of chemical reactions that just happen all the time. Why this one? $\endgroup$ Sep 11, 2019 at 10:17
  • $\begingroup$ What exactly do you mean? I want to understand the interactions between the molecules and how they form the end products of calcium carbonate and water. Like the oxygens of the hydroxide groups and co2 are partially negative etc, I just want to understand how their interactions come into play $\endgroup$
    – user83642
    Sep 11, 2019 at 10:36
  • $\begingroup$ You put your comment and more rather to the question itself, for the below: $\endgroup$
    – Poutnik
    Sep 12, 2019 at 6:39
  • $\begingroup$ We have a policy which states that ‎you should show your thoughts, effort and attempts to answer your question yourself. It'll make us certain that ‎we aren't doing your homework for you, and that the Q/A is beneficial for broad audience. As "homework class questions" are considered literal homeworks, self-study questions, puzzles, worked examples etc. Please edit in your full reasoning or thoughts on this. See Homework Otherwise, the question may get closed. $\endgroup$
    – Poutnik
    Sep 12, 2019 at 6:40
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    $\begingroup$ Oh alright, it’s not a homework question I’m just curious. Will edit. $\endgroup$
    – user83642
    Sep 12, 2019 at 7:19

1 Answer 1


The important thing to consider is the chemistry of carbon dioxide in water, forming carbonic acid. Note also that calcium hydroxide is at edge between being a strong or a weak base.

$$\begin{align}\ \ce{CO2 ^ + H2O &<=> CO2 . H2O} \tag{1} \\ \ce{CO2 . H2O &<=> H2CO3} \tag{2} \\ \ce{H2CO3 &<=> H+ + HCO3^-} \tag{3} \\ \ce{HCO3^- &<=> H+ + CO3^2- } \tag{4} \\ \ce{ Ca(OH)2 &<=> CaOH+ + OH- } \tag{5} \\ \ce{CaOH+ &<=> Ca^2+ + OH-} \tag{6} \\ \ce{Ca^2+ + CO3^2- &<=> CaCO3 v} \tag{7} \end{align}$$

Yes, bonds in $\ce{CO3^2-}$ are equivalent, with strength and length somewhere between a single and a double bond. It is similar as for $\ce{SO4^2-}$, $\ce{SO3^2-}$, $\ce{NO3^-}$, $\ce{ClO4^-}$, with sharing the "doublebondness" within the former two, and sharing the charge in all of them.


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