Water has bent structure (sp3, bond angle 104.5 degrees). High b.p. (100 degrees C), high specific heat, high dielectric constant (80.4) — all due to extensive H-bonding network. Anomalous expansion: maximum density at 4 degrees C (ice is less dense than water, floats — crucial for aquatic life). Hard water: Contains dissolved $Ca^{2+}$/$Mg^{2+}$ salts. Temporary hardness (Ca(HCO3)2, Mg(HCO3)2) — removed by boiling (decomposes to insoluble carbonate + CO2 + H2O) or Clark's method (adding Ca(OH)2). Permanent hardness (CaSO4, MgCl2, CaCl2) — removed by washing soda (Na2CO3), Calgon (Na6P6O18), zeolite (ion exchange), or synthetic ion-exchange resins.