How does salt soften hard water?

Hard water contains a lot of calcium or magnesium in it. A water softener replaces the calcium and magnesium ions in the water with sodium ions. The water in the house runs through a bed of small plastic beads or through a chemical matrix called zeolite. The beads or zeolite are covered with sodium ions. As the water flows past the sodium ions, they swap places with the calcium and magnesium ions.