Why does calcium carbonate decompose in sulphuric acid?

Here's the reaction:


$\ce{H2SO4(aq) + CaCO3(s) -> CaSO4(s) + H2O(l) + CO2(g)}$


I don't understand why calcium carbonate decomposes in sulphuric acid. It's not soluble in water. I could understand the hydrogen ions oxidizing the calcium, yet the reaction isn't a redox reaction; the oxidation states remain the same!





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> I don't understand why calcium carbonate decomposes... It's not soluble in water.
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It's more soluble in water than you might think. If it weren't soluble, we wouldn't have awesome things such as [stalagmites](https://en.wikipedia.org/wiki/Stalagmite) in caves.


Calcium sulfate is more soluble in water than the carbonate, but that isn't saying much. It's solubility is only about $\pu{2 g/L}$. So writing the product as $\ce{CaSO4(s)}$ is only correct if you are talking about dilute solutions. Anything too concentrated would decompose to form precipitated calcium sulfate.



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> I could understand the hydrogen ions oxidizing the calcium...
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Now *that* would be a very curious reaction. The calcium in calcium carbonate is already in the +2 oxidation state. Calcium has only two valence electrons and further oxidation would require disrupting an intact octet. It would be very unfavorable. $\ce{Ca^{2+} -> Ca^{3+} + e-}$ would be far too unfavorable a reaction for protons to carry out. $\ce{Ca^{3+}}$ would be a much stronger oxidant than aqueous protons.



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> Why does calcium carbonate decompose in sulphuric acid?
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The decomposition can still happen even with insoluble, solid-phase calcium carbonate because reactions can happen at surfaces. An acidic proton could be attacked by a solid-phase carbonate ion at the crystal surface, forming bicarbonate. The bicarbonate could attack another acidic proton, forming a surface-bound $\ce{H2CO3}$ molecule, which could spontaneously transform into $\ce{CO2}$ and water, which would be very loosely bound to the surface, allowing them to escape and exposing fresh not-yet-reacted calcium carbonate molecules to the aqueous surface, and the process begins anew...


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Since carbon dioxide is released out, leading to decrease in concentration of product ,hence the reaction goes in forward direction to attain equilibrium leading to breakdown of calcium carbonate. 


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