Explain the Ion Exchange water Softening

Softening is usually done using a resin (natural or synthetic long chain chemical with a sodium ion attached). When water containing hardness ions is passed through the resin, the hardness ions (Ca, Mg) exchange with Na ions on the resin (thus the resulting water is free of hardness ions but high in sodium).

When the sodium ions on the resin are exhausted, the resin is regenerated using a brine solution (which replaces all hardness ions attached to the resin with Na ions again) and the resin can be used again.