Cations are positively charged ions, such as calcium (Ca²⁺), magnesium (Mg²⁺) or sodium (Na⁺). They play a central role in water treatment , especially in water softening. Calcium and magnesium ions are the main cause of limescale in water, which can build up in pipes and appliances. In ion exchangers, these cations are exchanged for sodium ions to reduce water hardness and prevent limescale problems.

Here are examples of cations: