Britannica Sources used in this answer

How is salt used in water-softening equipment?

Created from Britannica articles using AI. AI can make mistakes, so verify using Britannica articles.

Cluster of translucent, white salt crystals with sharp cubic shapes viewed under a microscope at high magnification against a dark background. — salt Salt crystal magnified.

In water-softening equipment, common salt (sodium chloride) is used to regenerate the system's ion-exchange medium, such as zeolite or ion-exchange resin.12

During the softening process, these resins remove calcium and magnesium ions from the water by trading them for sodium ions. Over time, the resin becomes "exhausted" as its supply of sodium is depleted and replaced by the hardness-causing minerals.12

To restore the system, a concentrated solution of salt—often referred to as brine—is passed through the resin column. The high concentration of sodium ions in the salt solution displaces the accumulated calcium and magnesium ions, effectively "recharging" the resin so it can continue to soften water. Once the system is flushed with water to remove the displaced minerals, it is ready for further use.12