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Why does a water-softening resin release sodium ions into hard water?

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chemical structure of cation exchanger — ion-exchange reaction Figure 1: Chemical structure of cation exchanger. The exchangeable ions are marked ⁺. The whole structure is permeated by solvent molecules, usually water (not shown).

Water-softening resins release sodium ions into hard water as part of an ion-exchange reaction. These resins contain a supply of sodium ions that are held on the surface of the material.123

When hard water—which contains dissolved calcium and magnesium ions—passes through the resin, the resin selectively trades its sodium ions for the calcium and magnesium ions. This process effectively removes the hardness-causing minerals from the water by trapping them within the resin, while releasing an equivalent amount of sodium into the water supply.123

Once the resin's supply of sodium is exhausted, it can be regenerated by flushing it with a concentrated solution of common salt. The high concentration of sodium ions in the salt solution forces the calcium and magnesium ions off the resin, replacing them with fresh sodium ions so the process can begin again.12