Water Softening
Water containing a relatively high concentration of Ca2+, Mg2+ and other divalent cations is called hard water. Although the presence of these ions is generally not a health threat, they can make water unsuitable for some household and industrial uses. For example, these ions react with soaps to form an insoluble soap scum, the stuff of bathtub rings. In addition, mineral deposits may form when water containing these ions is heated. When water containing calcium ions and bicarbonate ions is heated, some carbon dioxide is driven off. As a result, the solution becomes less acidic, and insoluble calcium carbonate forms:Ca2+(aq)+ 2HCO3-(aq)→CaCO3(s) + CO2(g) + H2O(l) X6zdt%nuA
The solid CaCO3 coats the surface of hot-water systems and tea kettles, thereby reducing heating efficiency, These deposits, called scale, can be especially serious in boilers where water is heated under pressure in pipes running through a furnace. Formation of scale reduces the efficiency of heat transfer and reduces the flow of water through pipes.
The removal of the ions that cause hard water is called water softening. Not all municipal water supplies require water softening. In those that do, the water is generally taken from underground sources in which it has had considerable contact with limestone, CaCO3, and other minerals containing Ca2+, Mg2+, and Fe2+ The lime-soda process is used for large-scale municipal water-softening operations. The water is treated with lime, CaO [or slaked lime, Ca(OH)2], and soda ash, Na2CO3.*UUU#DU
These chemicals cause precipitation of Ca2+ as CaCO3 (Ksp =2.8 X 10-9) and Mg2+ as Mg(OH)2;, (Ksp= 1.8 X 10-11):
Ca2+(aq)+ CO32-(aq)→CaCO3(s)
Mg2+(aq) + 2OH(aq) →Mg(OH)2(s)6A0m*h&K~
Ion exchange is a typical household method for softening. In this procedure the hard water is passed through a bed of an ion exchange resin: plastic beads with covalently bound anion groups such as ―COO- or ―SO3-.
These negatively charged groups have Na+ ions attached to balance their charges. The Ca2+ ions and other cations in the hard water are attracted to the anionic groups and displace the lower-charged Na+ ions into the water. Thus, one type of ion is exchanged for another. To maintain charge balance, 2 Na+ ions enter the water for each Ca2+removed. If we represent the resin with its anionic site as R―COO- we can write the equation for the process as follows:
2Na(R―COO)(s) + Ca2+(aq) ==Ca(R―COO);(s) + 2Na+(aq)
Water softened in this way contains an increased concentration of Na+ ions. Although Na+ ions do not form precipitates or cause other problems associated with hard-water cations, individuals concerned with their sodium intake should avoid drinking water softened in this way. This includes people with high blood pressure (hypertension) or certain other health problems.When all the available Na+ ions have been displaced from the ion-exchange resin, the resin is regenerated by flushing it with a concentrated solution of NaCl. Homeowners accomplish this task by charging their units with large amounts of NaCl(s), which can be purchased in large bags at most grocery stores. Thehigh concentration of Na+ forces the equilibrium shown in the above equation to shift to the left, causing the Na+ ions to displace the hard-water cations, which are flushed down the drain.qFI.YPG5^ K
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