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.

Related Terms:

total hardness test