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Hard water is water that has high mineral content (in contrast with "soft water"). Hard water is formed when water percolates through deposits of limestone and chalk which are largely made up of calcium and magnesium carbonates. Hard drinking water may have moderate health benefits, but can pose serious problems in industrial settings, where water hardness is monitored to avoid costly breakdowns in boilers, cooling towers, and other equipment that handles water. In domestic settings, hard water is often indicated by a lack of suds formation when soap is agitated in water, and by the formation of limescale in kettles and water heaters.〔World Health Organization (Hardness in Drinking-Water ), 2003〕 Wherever water hardness is a concern, water softening is commonly used to reduce hard water's adverse effects. ==Sources of hardness== Water's hardness is determined by the concentration of multivalent cations in the water. Multivalent cations are positively charged metal complexes with a charge greater than 1+. Usually, the cations have the charge of 2+. Common cations found in hard water include Ca2+ and Mg2+. These ions enter a water supply by leaching from minerals within an aquifer. Common calcium-containing minerals are calcite and gypsum. A common magnesium mineral is dolomite (which also contains calcium). Rainwater and distilled water are soft, because they contain few ions.〔Hermann Weingärtner, "Water" in Ullmann's Encyclopedia of Industrial Chemistry, 2006(), Wiley–VCH, Weinheim. 〕 The following equilibrium reaction describes the dissolving and formation of calcium carbonate : :CaCO3 (s) + CO2 (aq) + H2O (l) (unicode:⇋) Ca2+ (aq) + 2HCO3− (aq) 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. Calcium and magnesium ions can sometimes be removed by water softeners.〔Christian Nitsch, Hans-Joachim Heitland, Horst Marsen, Hans-Joachim Schlüussler, "Cleansing Agents" in Ullmann’s Encyclopedia of Industrial Chemistry 2005, Wiley–VCH, Weinheim. 〕 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「hard water」の詳細全文を読む スポンサード リンク
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