The solubility of calcium carbonate CaCO3 in water at 25C is quite low. It is approximately 15 mg/L milligrams per liter or 15 ppm parts per million . This low solubility is due to the formation of a stable and insoluble solid when calcium ions Ca2+ react with carbonate ions CO32- in water. The solubility product constant Ksp for calcium carbonate at 25C is around 3.36 10^-8.