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Iron(II) carbonate is insoluble but in class you might not have seen any solid. The precipitate often redissolves because the iron(II) sulphate has acid added to it. Can you explain why the carbonate redissolves and how this can be described as an acid/base reaction

I know little to nothing about the topic and have done quite a bit of research but I can't find a simple definition that I am able to understand

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    – Poutnik
    Commented Mar 22, 2022 at 12:35

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Ferrons carbonate $\ce{FeCO3(s)}$ is like calcium carbonate $\ce{CaCO3}$ : it is soluble in carbonic acid with formation of the hydrogenocarbonate ion $\ce{HCO3^-}$in a reaction : $$\ce{FeCO3(s) + H2CO3 -> Fe^{2+} + 2 HCO3^- \tag{1}}$$ This means that $\ce{Fe(HCO3)2}$ is soluble in water.

$\ce{FeCO3(s)}$ can also be dissolved by the weak acidity produced by the hydrolysis of the $\ce{Fe^{2+}}$ ions coming from $\ce{FeSO4}$ in water, according to the two equations ($2$) and ($3$) : $$\ce{Fe^{2+} + H2O <=> [Fe(OH)]^+ + H^+ \tag{2}}$$ $$\ce{FeCO3 + H^+ -> Fe^{2+} + HCO3^- \tag{3}}$$

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  • $\begingroup$ $\ce{FeCO3}$ is an insoluble substance that does exist in water. $\ce{FeSO4}$ does not exist in solution, as it is soluble, and produces $\ce{Fe^{2+}}$ ions and $\ce{SO4^{2-}}$ ions. So the two reactions (1) and (3) of $\ce{FeCO3(s)}$ with acids do not occur with $\ce{Fe^{2+}}$ as it would mean that $\ce{Fe^{2+}}$ produces $\ce{Fe^{2+}}$ which is absurd. $\endgroup$
    – Maurice
    Commented Mar 22, 2022 at 15:00

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