Question

What ions are found in hard water? What happens when water is "softened"?

Short answer

Hard water is water that contains ${\text{Ca}}^{\text{2 +}}$and ${\text{Mg}}^{\text{2 +}}$ions. The elimination of dissolved hard water cations, which displace $N{a}^{+}$from a resin, "softens" the water.

Step-by-step solution

Definition of hard water.

The amount of dissolved calcium and magnesium in the water is the simplest definition of water hardness. Dissolved minerals, primarily calcium and magnesium, are abundant in hard water.

Determine the concentration.

The quantity of multivalent cations in the water determines its hardness.Water that contains large amounts of doubly charged cations, such as${\text{Ca}}^{2+}$ and$M{g}^{2+}$ is called hard water.

Ion exchanges.

The removal of hard-water cations, called water softening, is accomplished by exchanging $N{a}^{+}$ions for ${\text{Ca}}^{2+}$and $M{g}^{2+}$ions.

A typical domestic system for ion exchange contains an ion exchange resin, an insoluble polymer that has covalently bonded anion groups, such as $\text{- S}{{\text{O}}_3}^{-}$or ,$-{\text{COO}}^{-}$to which $N{a}^{+}$ions are attached to balance the charges.

The divalent cations in hard water are attracted to the resin’s anionic groups and displace the $N{a}^{+}$ions into the water.

Therefore, Water containing ${\text{Ca}}^{2+}$and $M{g}^{2+}$ions is called hard water. Water is "softened" by the removal of dissolved hard water cations, which displace $N{a}^{+}$from a resin.