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Water has a bent structure (sp3, 104.5 degrees), extensive H-bonding, high boiling point (100 C), high specific heat, and a dielectric constant of 80.4 (excellent ionic solvent). Its anomalous maximum density at 4 C means ice floats — critical for aquatic ecosystems.

Hard water contains dissolved $Ca^{2+}$/$Mg^{2+}$ salts in two categories: temporary hardness (Ca(HCO3)2, Mg(HCO3)2) and permanent hardness (CaSO4, MgCl2, CaCl2). Temporary hardness is removed by boiling (bicarbonate → carbonate + CO2) or Clark's method (adding Ca(OH)2). Permanent hardness requires Na2CO3 (precipitates CaCO3), Calgon (Na6P6O18, sequesters $Ca^{2+}$), zeolite (ion exchange: Na2Z + $Ca^{2+}$ → CaZ + 2$Na^+$), or synthetic ion-exchange resins.

The distinction matters practically: boiling only removes temporary hardness. Na2CO3 and ion exchange treat both types. Zeolites are regenerated with NaCl solution. Modern demineralisation uses both cation ($H^+$ form) and anion ($OH^-$ form) resins to produce ultra-pure deionised water.