Question

why should be insoluble in water CaCO3

Answer 1

In order for an ionic substance to dissolve in water, its ions have to be solvated by the water molecules and the ions separated in water have to have a lower free energy than they do when in a solid form.

Calcium carbonate is insoluble in water because it is extremely stable as a solid and water doesn't have sufficient solvating capability to cause the ions to separate and come into solution.

Both calcium and carbonate do have salts which can dissolve in water.

Both sodium carbonate and calcium chloride are water soluble. However, relative to their water solutions, solid sodium carbonate and solid calcium chloride are much less stable than the water solution. So the ions will go into solution.

So, as I mentioned above, calcium carbonate forms a particularly stable solid and will not dissolve in water.

The attraction in this case of solid calcium carbonate is an ionic attraction between the calcium ions and the carbonate ions.