A resin or polymer that serves as a medium for ion exchange is known as an ion-exchange resin or ion-exchange polymer. It is an insoluble matrix (or support structure) made from an organic polymer substrate, typically appearing as tiny (0.25-1.43 mm radius) microbeads that are white or yellowish in color. The process is known as ion exchange because the beads are often porous, providing a wide surface area on and inside them where the trapping of ions takes place along with the concomitant release of other ions. Ion-exchange resin comes in many different varieties. Polystyrene sulfonate is the main ingredient in most commercial resins.