Softening of hard water is required if level of hardness exceeds: March 2011
Ans. B : 150 mg/L 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 dissolving/formation 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 mass/volume (mg/L) 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 meq/1 Soft water: Less than 1 (< 50 mg/1) Moderately hard: 1-3 (50-15 mg/1) Hard water: 3-6 (150-300 mg/1) Very hard water: Over 6 (> 300 mg/1) 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 meq/1 or 150 mg/1).
50 mg/L
150 mg/L
200 mg/L
250 mg/L