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Answer.

Because of the problem it causes, hard water is often softened for use in factories, industries and homes. That means removing the dissolved calcium and magnesium ions. Described below are the methods of treating and purifying hard water.

(a)Boiling: Boiling removes temporary hardness in water, as you saw early. Boiling causes calcium carbonate to precipitate. The hydrogencarbonate in water are decomposed to carbonates, which are insoluble in water
In this way, the calcium is removed, since the calcium carbonates being insoluble, takes no further part in the reaction. An insoluble calcium salt cannot cause hardness. However, this method uses a lot of fuel, which makes it expensive to do on a large scale.

(b) Distillation; Distillation removes all impurities from water. This gets rid of both temporary and permanent hardness. In distillation, the water is boiled and the steam collected, cooled and condensed. Distilled water is pure and softest water. All the dissolved substances have been removed. Like boiling, it is an expensive option in terms of fuel used. But it is essential for some purposes, for example for laboratory experiments and for making drugs.

(c) Addition of calcium hydroxide: Addition of calculated amounts of calcium hydroxide can remove temporary hardness. The quantity to be added should be properly calculated because excess would cause hardness on its own account. The amount of calcium hydroxide to be added is calculated based on knowledge of the hardness of water and the capacity of the reservoir (Clark’s method). The calcium hydroxide reacts with the hydrogencarbonates dissolved in water and precipitates as the insoluble calcium carbonates.

Ca(OH)2(s) slightly soluble + Ca(HCO3)2(aq) → 2CaCO3(s) + 2H2O(l)insoluble

(d) Addition of sodium carbonate (washing soda): Washing soda removes both temporary and permanent hardness by precipitating calcium carbonate. It reacts with calcium hydrogencarbonate (which causes temporary hardness) to form sodium hydrogencarbonate like this:

Na2CO3(aq)+ Ca(HCO3)2(aq)→ 2NaHCO3(aq))+ CaCO3(s)

It also reacts with calcium sulphate (which causes permanent hardness) to form sodium sulphate.

Na2CO3(aq)+ CaSO4(aq)→ CaCO3(s)+ Na2SO4(aq)

These sodium salts are soluble, but do not cause the water to be hard. The calcium and magnesium ions are precipitated as insoluble calcium and magnesium carbonates. Ionically, the situation is like this:

Ca2+(aq)+ CO32-(aq)→ CaCO3(s)
Mg2+(aq)+ CO32-(aq)→ MgCO3(s)

(e) Ion Exchange; Another method that removes both temporary and permanent hardness in water is the use of ion exchange resin. A typical ion exchanger is a container full of small beads. These beads are made of special plastic called ion exchange resin. The resin beads are porous and contain sodium ions. When hard water flows through the resin, the calcium and magnesium ions in the water are exchanged for the sodium ions and attach themselves to the resin. This process, therefore, removes calcium and magnesium ions from the water. They are replaced by sodium ions, which do not make the water hard.

An ion exchange column removes ca2+ and mg2+ ions from the water and replaces them with Na+ ions
When all sodium ions have been removed from the resin, it is regenerated by pouring a concentrated solution of sodium chloride through it. The sodium ions remove the calcium and/or magnesium ions off the resin and the ion exchanger is ready for the use again.Other ions could also be used instead of sodium for the resin. But sodium chloride is normally used to supply the sodium ions because salt is cheap.

(f) Use of softeners; Many modern washing powders now have softeners added to them. The softeners are often phosphates. The phosphates ions react with calcium ions to form calcium phosphate and remove the hardness.

3Ca2+(aq)+ 2PO43-(aq)→ Ca3(PO4)2(s)

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