Calcium carbonate latter is especially evident in day-to-day life of people who have hard water. Water in aquifers underground can be exposed to levels of CO much higher than atmospheric. As such water percolates through calcium carbonate rock, the CaCO dissolves according to the second trend. When that same water then emerges from the tap, in time it comes into equilibrium with CO levels in the air by outgassing its excess CO. The calcium carbonate becomes less soluble as a result and the excess precipitates as lime scale. This same process is responsible for the formation of stalactites and stalagmites in limestone
Carbonate hardness carbonate expressed as if calcium carbonate were the sole source of carbonate ions. An aqueous solution containing 120 mg NaHCO (baking soda) per litre of water will contain 1.4285 mmol/l of bicarbonate, since the molar mass of baking soda is 84.007 g/mol. This is equivalent in carbonate hardness to a solution containing 0.71423 mmol/L of (calcium) carbonate, or 71.485 mg/l of calcium carbonate (molar mass 100.09 g/mol). Since one degree KH = 17.848 mg/L CaCO, this solution has a KH of 4.0052 degrees. Carbonate hardness should not be confused with a similar measure Carbonate Alkalinity which is expressed in either