<P> The reaction can go in either direction . Rain containing dissolved carbon dioxide can react with calcium carbonate and carry calcium ions away with it . The calcium carbonate may be re-deposited as calcite as the carbon dioxide is lost to atmosphere, sometimes forming stalactites and stalagmites . </P> <P> Calcium and magnesium ions can sometimes be removed by water softeners . </P> <P> Temporary hardness is a type of water hardness caused by the presence of dissolved bicarbonate minerals (calcium bicarbonate and magnesium bicarbonate). When dissolved, these minerals yield calcium and magnesium cations (Ca, Mg) and carbonate and bicarbonate anions (CO, HCO). The presence of the metal cations makes the water hard . However, unlike the permanent hardness caused by sulphate and chloride compounds, this "temporary" hardness can be reduced either by boiling the water, or by the addition of lime (calcium hydroxide) through the process of lime softening . Boiling promotes the formation of carbonate from the bicarbonate and precipitates calcium carbonate out of solution, leaving water that is softer upon cooling . </P> <P> Permanent hardness is hardness (mineral content) that cannot be removed by boiling . When this is the case, it is usually caused by the presence of calcium sulphate / calcium chloride and / or magnesium sulphate / magnesium chloride in the water, which do not precipitate out as the temperature increases . Ions causing permanent hardness of water can be removed using a water softener, or ion exchange column . </P>

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