Final answer:
Water softening involves the reduction of calcium and magnesium ions in hard water and substitution with sodium ions via ion-exchange, lime softening, reverse osmosis, or chelation.
Step-by-step explanation:
Water softening is accomplished by reducing hardness ions such as calcium (Ca2+) and magnesium (Mg2+) and replacing them with more acceptable ions, commonly sodium (Na+). The process is primarily achieved through ion-exchange resins or lime softening, and can also involve reverse osmosis or chelation.
During ion-exchange, hard water passes through a resin containing sodium ions. As water flows over the resin, calcium and magnesium ions are replaced by sodium ions. The replaced ions bind to the resin, resulting in softened water with fewer dissolved minerals, although slightly saltier in taste.
In lime softening, Ca(OH)2 is added to water, causing calcium and magnesium to precipitate as CaCO3 and Mg(OH)2, which can be filtered out. Reverse osmosis pushes water through a semipermeable membrane, filtering out larger hardness ions. Lastly, chelating agents like EDTA bind the hardness ions, preventing them from interfering with soaps and detergents.