Final answer:
A water softener releases sodium ions (Na+) into a water stream to replace calcium and magnesium ions, thus softening the water.
Step-by-step explanation:
During normal operation, a water softener releases sodium ions (Na+) into a water stream. This is achieved through a process known as ion exchange, where hard water containing calcium (Ca2+) and magnesium (Mg2+) ions is passed over ion-exchange resins. These resins are composed of complex sodium salts and function by exchanging the sodium ions for the undesired hardness-causing ions. Consequently, the hard water ions are fixed to the resin, replacing them with sodium ions and resulting in softened water that is easier to use for cleaning and does not produce scale accumulation in pipes and fixtures.