Question

Polyphosphates are used as a water softening agents because they

• A. Form soluble complexes with anionic species
• B. Precipitate anionic species
• C. Form soluble complexes with cationic species
• D. Precipitate cationic species

Answer 1

The correct option is C

Form soluble complexes with cationic species

Explanation of correct option.

Option (C) Form soluble complexes with cationic species:

• Polyphosphates are the salts or esters of polymeric Oxyanions formed from tetrahedral Phosphate (${\mathrm{PO}}_4^{3-}$) structural units linked together by sharing oxygen atoms.
• Polyphosphates are applicable as water softening agents because they form soluble complexes with cationic species of hard water, which is the cause of the hardness of the water.
• The hardness of the water is mainly caused by the presence of naturally occurring dissolved calcium and magnesium ions.
• Water softening is the process of removing the dissolved calcium and magnesium salts that causes the hardness of the water.
• The reaction of the water softening process in the presence of polyphosphate- A water softening agent is: $\begin{array}{r}\underset{\mathrm{Calcium}\mathrm{sulfate}}{2{\mathrm{CaSO}}_4}+\underset{\mathrm{Sodium}\mathrm{hexametaphosphate}}{{\mathrm{Na}}_2\left[{\mathrm{Na}}_4{\left({\mathrm{PO}}_3\right)}_6\right]}\to \underset{\mathrm{Disodium}\mathrm{calcium}\mathrm{hexametaphosphate}}{{\mathrm{Na}}_2\left[{\mathrm{Ca}}_2{\left({\mathrm{PO}}_3\right)}_6\right]}+\underset{\mathrm{Sodium}\mathrm{sulphate}}{2{\mathrm{Na}}_2{\mathrm{SO}}_4}\\ \underset{\mathrm{Magnesium}\mathrm{sulfate}}{2{\mathrm{MgSO}}_4}+\underset{\mathrm{Sodium}\mathrm{hexametaphosphate}}{{\mathrm{Na}}_2\left[{\mathrm{Na}}_4{\left({\mathrm{PO}}_3\right)}_6\right]}\to \underset{\mathrm{Disodium}\mathrm{magnesium}\mathrm{hexametaphosphate}}{{\mathrm{Na}}_2\left[{\mathrm{Mg}}_2{\left({\mathrm{PO}}_3\right)}_6\right]}+\underset{\mathrm{Sodium}\mathrm{sulphate}}{2{\mathrm{Na}}_2{\mathrm{SO}}_4}\end{array}$