Hardness (mg/L CaCO₃) = 2.497 × [Ca²⁺] + 4.118 × [Mg²⁺]
Water hardness measures dissolved Ca²⁺ and Mg²⁺ ions, expressed as mg/L CaCO₃ equivalents. The conversion factors (2.497 for Ca, 4.118 for Mg) account for the different molecular weights. Classification (USGS): soft <60, moderately hard 60-120, hard 120-180, very hard >180 mg/L. Hard water causes: scale buildup in pipes and heaters (CaCO₃ precipitation), soap scum (insoluble calcium stearate), spotted dishes, and reduced detergent efficiency. Softening methods: ion exchange (water softeners — Na⁺ replaces Ca²⁺/Mg²⁺), lime softening (adds Ca(OH)₂ to precipitate CaCO₃), and reverse osmosis. Temporary hardness (bicarbonates) is removed by boiling; permanent hardness (sulfates, chlorides) requires chemical treatment.