Water treatment resin is divided into cationic resin and anionic resin. Cationic resin is further subdivided into sodium type and hydrogen type. Sodium type resin exchanges calcium and magnesium ions in the water into sodium ions, making the water soft. Hydrogen resin exchanges calcium and magnesium ions in water into hydrogen ions to soften the water. Anion resin contains replaceable hydroxide ions, which can replace acid ions in water. Using anion resin and hydrogen cation resin at the same time can soften the water. Water becomes pure water. In the water treatment industry, ion exchange is the reaction of equal charge moles between ions in the water and ions on the ion exchange resin.