Question

Permanent hardness of water can be removed by
A. permutit method
B. boiling
C. ion exchange
D. calgon’s process

Answer 1

Hint:The hard water contains a high amount of minerals. The main salts present in hard water are carbonates and bicarbonates of alkaline earth metals. Hard water has a multivalent cation. The commonly present cations are calcium${\text{C}}{{\text{a}}^{{\text{2 + }}}}$ and magnesium${\text{M}}{{\text{g}}^{{\text{2 + }}}}$. The ions enter into the water for leaching. The ions are mixed into the water in the form of their minerals.

Complete answer
Based on the concentration of ions, particularly calcium and magnesium, the water is divided into two parts: Hard water and soft water.

Permuted method is used for filtration of hard water. Filtration removes the impurities. Permutit is an artificial zeolite. Permutit has a porous structure. The pores can trap ions of specific size.
The permutit method is a simple filtration method. In this method zeolite that is also known as natural sand is used to filter the hard water.

Hard water is passed through sodium permutit. Permutit is a hydrated sodium alumina-orthosilicate. The formula of permutit is ${\text{N}}{{\text{a}}_{\text{2}}}{\text{A}}{{\text{l}}_{\text{2}}}{\text{S}}{{\text{i}}_{\text{2}}}{{\text{O}}_{\text{8}}}{\text{.x}}{{\text{H}}_{\text{2}}}{\text{O}}$. It has a porous structure.
In this method, permutit is taken into a cylinder and hard water is passed over it. Ions get exchanged. The calcium and magnesium get trapped in zeolite and sodium ion passes from the zeolite to water thus, hard water becomes soft as the calcium and magnesium ion removes from water and sodium permutit converts into calcium and magnesium permutit. Boiling can remove hydrogen carbonate but cannot remove hardness due to calcium and magnesium ions.

The hard water can be converted into soft water by the ion exchange method. Water is passed over man-made resin which contains the sodium ions. When hard water passes over it, the calcium and magnesium ions are exchanged by sodium ions.

The complex ${\left( {{\text{NaP}}{{\text{O}}_{\text{3}}}} \right)_6}$ is known as calgon. On dissociation it gives sodium ions which are replaced with calcium and magnesium ions.
\[{\left( {{\text{NaP}}{{\text{O}}_{\text{3}}}} \right)_6}\, \to 2\,{\text{N}}{{\text{a}}^ + }\, + \,{\text{N}}{{\text{a}}_{\text{4}}}{{\text{P}}_{\text{6}}}{\text{O}}_{18}^{2 - }\]
\[{\text{C}}{{\text{a}}^{2 + }}\, + \,{\text{N}}{{\text{a}}_{\text{4}}}{{\text{P}}_{\text{6}}}{\text{O}}_{18}^{2 - } \to {\text{CaN}}{{\text{a}}_{\text{4}}}{{\text{P}}_{\text{6}}}{{\text{O}}_{18}}\]

Therefore, option (A) permutit method, (C) ion exchange and (D) calgon’s process are correct.

Note:Zeolites are a shape-selective catalyst. Zeolites have cavities of specific size, so they can trap ions according to the size of the cavity. During the permuted method, the zeolite permutit gets deactivated so it regenerates by passing a concentrated brine solution over it. So, calcium and magnesium ions get converted into chloride, by washing these chlorides removes and regenerates the permutit. In methods, generally ions producing hardness get exchanged with sodium ions.