A water softener is used to soften hard water by removing the minerals that cause the water to be hard.

Water softeners are specific ion exchangers that are designed to remove ions, which are positively charged (mainly calcium (Ca2+) and magnesium (Mg2+).

Ion exchange resins are very small porous round plastic beads. The polymer structure of the resin bead contains a fixed negative ion that is permanently attached. This cannot be removed. Each negatively charged exchange site can hold a positively charged ion. In this case, sodium which has a positive charge is attached to the exchange site (negative and positive charges attract each other).

The calcium and magnesium ions present in hard water have a stronger positive charge than the sodium ions. As a result the calcium and magnesium have a stronger attraction to negatively charged resin bead than sodium does. Hence when hard water is passed through the resin bead, the sodium ion is kicked off the resin bead and the calcium and magnesium ions take its place and get attached to the bead. In simple terms we can say that the calcium and magnesium ions are exchanged for sodium ions. Hence the hard water turns into soft water since the treated water does not contain any calcium or magnesium ions.

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