What is water softening? Water softening is replacing of magnesium, calcium and other scale causing cations with sodium ions. What is water softener? • Water softeners are ion exchangers that are designed to remove ions, which are positively charged. • A water softener consists of a pressure vessel containing cation exchange resin. • Adsorption takes place over the surface of the resin, as the hard water passes through it. • Resin adsorbs calcium and magnesium ions and gives sodium ions which do not constitute to hardness. i.e. • Once the resin gets saturated with Ca++ & Mg++ ions, it is regenerated with NaCl to again make it ready for softening.