Ion exchange is a water treatment process commonly used for water softening or demineralisation. It can also be used to remove other substances such as nitrates, fluorides, sulphates and arsenic.

Ion exchange describes a specific chemical process in which unwanted dissolved ions are exchanged for other ions with a similar charge. Ions are atoms or molecules containing a total number of electrons that are not equal to the total number of protons and consequently carry a net charge.

In the exchange process during water treatment, positively charged ions that come into contact with the ion exchange resin are exchanged with positively charged ions available on the resin surface.

For water softening the replacement ions are usually sodium. In effect this means that harmful Calcium ions are replaced with Sodium ions. Since Calcium is the the source of limescale, replacing these opens softens the water. The exchange process occurs between a solid (usually a resin) and the water. The resin is preloaded with sodium ions. The resin is typically used in the form of small beads which allow the water to flow between them and also helps to maximises surface area coming to contact with the untreated water.