<prompt> Softening of hard water is required if level of hardness exceeds: March 201150 mgL150 mgL200 mgL250 mgL  <response>: Ans. B : 150 mgL Softening of the water is recommended at level of hardness of more than 150 mg liter of calcium carbonate 3 meq liter Hardness in water It is defined as concentration of multivalent cations. Multivalent cations are cations positively charged metal complexes with a charge greater than 1. They mainly have the charge of 2. These cations include Ca2 and Mg2. Common calcium-containing minerals are calcite and gypsum. A common magnesium mineral is dolomite which also contains calcium. The following equilibrium reaction describes the dissolvingformation of calcium carbonate scales: CaCO3 CO2 H20 Ca2 2HCO3- The total water hardness, including both Ca2 and Mg2 ions, is repoed in pas per million ppm or massvolume mgL of calcium carbonate CaCO3 in the water. Although water hardness usually measures only the total concentrations of calcium and magnesium the two most prevalent divalent metal ions, iron, aluminium, and manganese can also be present at elevated levels in some locations. The presence of iron characteristically confers a brownish rust-like colour to the calcification, instead of white the color of most of the other compounds. Classification and level of hardness in meq1 Soft water: Less than 1 50 mg1 Moderately hard: 1-3 50-15 mg1 Hard water: 3-6 150-300 mg1 Very hard water: Over 6 300 mg1 Water softening is the reduction of the concentration of calcium, magnesium, and ceain other metal cations in hard water. These hardness ions can cause a variety of undesired effects including interfering with the action of soaps, the build up of limescale, which can foul plumbing, and galvanic corrosion. Conventional water-softening appliances intended for household use depend on an ion-exchange resin in which hardness ions are exchanged for sodium ions. Water softening may be desirable where the source of water is hard when the hardness exceeds 3 meq1 or 150 mg1. 