We can use a cops and robbers analogy for ionic solutions. When the density of cops per unit area, times the density of robbers per unit area, exceeds some value, they'll encounter each other often enough for the number of arrests to equal the number of escapes. When that happens the system is in equilibrium. In the model above, the cops represent carbonate ions and the robbers represent calcium ions. If calcium carbonate behaved like this, it would behave pretty much like gypsum and be weakly soluble in water.
However, there's another ionic species around that can react with carbonate and that's hydrogen. Hydrogen ions are what make things acidic, so to model the hydrogens, we need something corrosive and caustic, something that eats away at, corrupts and destroys everything it touches.
We'll let lawyers represent hydrogen ions.
When hydrogen ions combine with carbonate, they form the bicarbonate ion. They're not carbonate any more, therefore more calcium carbonate can go into solution.
In the model above, the lawyers ride herd on the cops so tightly the cops are ineffective
Converting carbonate ions to bicarbonate takes a carbonate ion out of limestone and also allows a calcium to escape into solution.
What really enhances the solubility of calcium carbonate is that the atmosphere is constantly supplying fresh carbon dioxide which forms carbonic acid (H2CO3). That's a convenient chemical fiction: actually less than one per cent of the dissolved carbon dioxide actually forms carbonic acid. What does matter is that the dissolved carbon dioxide reacts with water to form bicarbonate ions and liberate a hydrogen ion. This extra hydrogen is then available to combine with a carbonate ion in limestone, forming a second bicarbonate ion and allowing a calcium ion to enter solution as well.
It's a bit analogous to the question how come a thin film of oxide protects aluminum from corrosion, whereas rust not only doesn't protect iron but actually accelerates corrosion. Aluminum oxide and ferric oxide both have exactly the same atomic structures, differing only in their cations.
In both cases, it's the presence of an intermediate state. In the case of iron, if we have Fe+++ in contact with metallic iron (Fe0), we have the reaction 2Fe+++ + Fe0 = 3Fe++. Each Fe+++ strips an electron off the Fe0 atom, oxidizing it to Fe++ while reducing the ferric ions to ferrous as well. Then both can combine with oxygen, become fully oxidized, and the stage is set for the next cycle.
Created 03 April 2006, Last Update 14 December 2009
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