One of three divalent cations that can be present in water, including calcium (Ca⁺²), magnesium (Mg⁺²) and ferrous (Fe⁺², a form of iron). Hardness ions develop from dissolved minerals, bicarbonate, carbonate, sulfate and chloride. Bicarbonate salts cause temporary hardness, which can be removed by boiling the water and leaving behind a calcium carbonate solid. Mg⁺² and Fe⁺² ions can be removed by raising the pH (with NaOH or KOH) and then allowing the precipitated Fe(OH)₂ and Mg(OH)₂ to settle out. Calcium hardness can be removed by adding excess sodium carbonate to precipitate Ca⁺² as CaCO₃. Hard water can be passed through an ion exchange column where hardness ions are captured on the resin. Removal of hardness is the process called water softening.