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.

Examples :

                                            Ion Exchange Process

                          Water Softener in Running Mode ( Service mode)

Water Softener – Service Mode When the water softener is providing soft water, it is said to be in Service mode.

During this Service cycle, water first enters the resin filled vessel from the top. The cations or the Calcium and Magnesium ions (hardness minerals) are immediately attracted towards the resin where the sodium ions are exchanged for the calcium and magnesium ions. Soft water flows out from the outlet.

This ion exchange takes place layer wise.(see diagram below) The top layer gets depleted of the sodium ions first followed by the lower layers.
Slowly the entire bed of resin is depleted of sodium ions since all resin beads get covered with calcium and magnesium ions.
Now It is time to regenerate the water softener resins.

Water Softener – Regeneration Mode Eventually the entire resin beads become saturated with hardness mineral cations of calcium and magnesium. At this point the water softener resin beads need to be replenished with sodium ions.

This process of replenishing the resin beads with sodium ions is known as regeneration.

During this process, the resin beads are washed with a strong salt water solution. This salt water solution is known as Brine.
As the brine gets in contact with the resin beads, the sodium in the brine forces the calcium and magnesium to be released and discharged as waste.
After sometime the entire beads are once again coated completely with sodium ions.