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How does an ion-exchange resin soften hard water?

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Ion-exchange resins soften hard water by removing dissolved calcium and magnesium salts, which are responsible for water hardness.1

The process involves passing hard water through a column containing a synthetic or natural ion-exchange resin. These resins contain charged sites that attract ions from the surrounding solution. Specifically, the resin acts as a cation exchanger, trading sodium ions for the calcium and magnesium ions present in the water.21

As the water flows through the resin bed, the resin captures the hardness-causing ions, effectively softening the water. Over time, the resin becomes saturated with calcium and magnesium. To restore its functionality, the column is regenerated by passing a concentrated solution of common salt through it; the excess sodium ions displace the accumulated calcium and magnesium ions, allowing the resin to be reused.1