Water hardness is a measure of the concentration of dissolved calcium and magnesium ions in water. Hard water can cause scale buildup in pipes and reduce the effectiveness of soaps and detergents.
Water hardness is caused primarily by dissolved calcium (Ca2+) and magnesium (Mg2+) ions, which enter water through the dissolution of limestone, dolomite, and other mineral-bearing rocks. Hardness is typically expressed as milligrams per liter of calcium carbonate (CaCO3) equivalent. Water is classified as soft (0-60 mg/L), moderately hard (61-120 mg/L), hard (121-180 mg/L), or very hard (above 180 mg/L). Hard water does not pose health risks and may actually provide beneficial dietary minerals, but it creates practical problems including scale deposits in water heaters, boilers, and plumbing, reduced lathering of soaps, and spots on dishes and glassware. Temporary hardness is caused by dissolved bicarbonates and can be removed by boiling, while permanent hardness is caused by sulfates and chlorides and requires ion exchange or reverse osmosis for removal. Water softening systems typically use ion exchange resins that replace calcium and magnesium ions with sodium ions. Hardness is an important parameter in industrial water treatment, aquaculture, and household water quality assessment.