Ion exchange is a process that includes two very similar applications of the same technology. The first is water softening. This is the process of removing ions from the water and replacing them with sodium ions and chloride ions. The most common use for this is residential, where a homeowner is trying to reduce the hardness or improve the taste of the water that the municipal service provides. This also reduces deposits and scale that can be left from water with a high level of hardness. The second application is deionization. In deionization, the hardness and other ions that are initially in the water are removed and replaced with H⁺ and OH^- ions, which can combine to form water. This is used in applications where extremely pure water is required.

The process works like this: Ion exchange resins (little beads that are charged) are coated with the replacement ions. In the case of water softening the beads are coated with Na⁺ and Cl^-. In the case of deionization, they are coated with H⁺ and OH^-. Water flows over the resin. The ions in the water are attracted to the resin. The ions in the water attach themselves to the resin, and knock off the ions that are already attached.

The resin is exhausted when all of the replacement ions are gone. In order to replenish the resin, also called regenerating the resin, a strong solution of the replenishment ions must be applied to the resin. This removes the ions that came from the water and regenerates the resin. The solution that is used to regenerate the water softeners is concentrated salt water called brine. There are two solutions that are used to regenerate a deionizer. One is a concentrated acid, and the other is a concentrated base.