During water softening by ion exchange, cation exchange resins are used to replace cations such as calcium and magnesium ions, which are responsible for water hardness, with sodium ions,
using a strongly acidic cation exchanger in the sodium form. This cation exchanger is loaded with sodium ions and exchanges these with an equivalent amount of ions of the hardness components calcium and magnesium. Because one salt is merely exchanged for another during the water softening process, this is also referred to as a neutral exchanger. The water softening process can be reversed for regeneration by providing the ion exchanger with an excess of sodium ions. A saturated salt solution, which can be prepared by dissolving salt tablets, is used for regeneration. Larger water softening systems often use Big Bags.