why is calcium carbonate insoluble in regular water?

Answer 1

It ISN'T insoluble. The solubility product of calcium carbonate is K=4.47 x10^9 at 25 C. Now, I'll grant you that's not a ton of solubility, but that's not exactly insolubility, either.

You can get rid of more of the stuff by causing a reaction with carbonate (CO3 2-) and hydrogen ions which turns it into carbon dioxide and water... but that's really more a matter of getting rid of the dissolved products rather than dissolving more of it.

Likewise, calcium carbonate will react with carbon dioxide saturated water to form calcium bicarbonate, which is one of the primary means of eroding carbonate rocks.

As to WHY it's not more soluble one way to make a good guess would be to look at other similar molecules. As it turns out carbonate is largely insoluble when paired with any cation stronger than +1, but largely soluble otherwise. There's your hint. What's probably happening is that the ionic bond simply becomes too strong with the larger cations to be easily seperated by just weakly polar water. If you use a different, MORE polar solvent, it should (if our guess is true) dissolve much better.

Hope that helps!

Answer 2

Regular water is neutral, calcium carbonate is an alkaline. to react with calcium carbonate it needs to be something acidic.