which is more soluble than . Note that for P = 10−12 atm, the product is still below the solubility product of (). For still lower pressure, precipitation will occur before precipitation. As ambient partial pressure increases to levels above atmospheric, pH drops, and much of the carbonate ion is converted to bicarbonate ion, which results in higher solubility of . The effect of the 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 much higher than atmospheric. As such water percolates through calcium carbonate rock, the dissolves according to the second trend. When that same water then emerges from the tap, in time it comes into equilibrium with levels in the air by outgassing its excess . 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 caves. Two hydrated phases of calcium carbonate, monohydrocalcite and ikaite , may precipitate from water at ambient conditions and persist as metastable phases. With varying pH, temperature and salinity: scaling in swimming pools In contrast to the open equilibrium scenario above, many swimming pools are managed by addition of sodium bicarbonate () to about 2 mM as a buffer, then control of pH through use of HCl, , , NaOH or chlorine formulations that are acidic or basic. In this situation, dissolved inorganic carbon (total inorganic carbon) is far from equilibrium with atmospheric . Progress towards equilibrium through outgassing of is slowed by In this situation, the dissociation constants for the much faster reactions allow the prediction of concentrations of each dissolved inorganic carbon species in solution, from the added concentration of (which constitutes more than 90% of Bjerrum plot species from pH 7 to pH 8 at 25 °C in fresh water). Addition of will increase concentration at any pH. Rearranging the equations given above, we can see that = , and [] = . Therefore, when concentration is known, the maximum concentration of ions before scaling through precipitation can be predicted from the formula: []max = × The solubility product for (Ksp) and the dissociation constants for the dissolved inorganic carbon species (including Ka2) are all substantially affected by temperature and salinity, with the overall effect that []max increases from freshwater to saltwater, and decreases with