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I am curious why a discrete covalent molecular substance, say CO2, would react with water to form an acid, what happens in such a reaction? I'm assuming it has something to do with disassociated ions in the water but why would CO2 want to bond?

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    $\begingroup$ Stability depends on the environment. Many many substances are just fine around themselves, mas will promply react when exposed to other substances. $\endgroup$ Commented Jan 17, 2023 at 13:54
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    $\begingroup$ Stability and reactivity are two different concepts. Elementary fluorine is infinitely stable as it does not undergo decomposition. Yet it is extremely reactive. // As stability is usually meant the tendency not to undergo decomposition in pure form or in defined environment. // As reactivity is usually meant the tendency to react in given environment with explicit reagents or reagent classes. $\endgroup$
    – Poutnik
    Commented Jan 17, 2023 at 14:00

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What you meant by "stable" is relative. It is not some absolute. First we need to define "stability".

From what you wrote, I assume by "stable" you mean compounds that don't participate in chemical reaction in a normal environment. We need to define "normal environment" as well. We may define it as STP. But that is not absolute and so, "stability" will be relative.

Now, how do we know that CO2 is stable? One may say, well, it doesn't react with many elements/ compounds. For example it doesn't usually react with dilute hydrochloric acid in normal environment. However, if we give enough hea, the temperature will rise enough that it will eventually react. Why will it react then?

Before giving heat, the sum of the bond energies of the products were more than the sum of the bond the bond energies of the reactants. So, Δh was positive. After giving heat and the heat energy being greater than Δh, the reaction starts.

When CO2 reacts with water, Δh is not positive, it's negative, and thus it will react. Here, carbon di oxide is stable but it will be more stable after becoming H2CO3 as it will have less bond energy and in our universe having the lowest energy usually leads to the most stable state. However, it won't become more stable after reacting with most other compounds. That's why it's relatively stable.

Again, stability is relative. Unstable compound will become more stable with the temperature drop, stable elements become unstable with the temperature increasing (relatively speaking).

Hope this helps!

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    $\begingroup$ Note that carbonic acid forms less than 1 % of dissolved carbon dioxide which stays mostly in form of hydrated oxide. $\endgroup$
    – Poutnik
    Commented Jan 17, 2023 at 16:27
  • $\begingroup$ Can't help but wonder how you imagine its reaction with HCl... Other than that, you should check your post for typos and don't use bold too much. $\endgroup$
    – Mithoron
    Commented Jan 19, 2023 at 2:11

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