Question

Polyphosphate are used as 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

Hint: Hard water is defined as the water that has high mineral content (in comparison with "soft water"). Hard water has been formed by deposits of limestone, chalk or gypsum which are largely composite of calcium and magnesium carbonates, bicarbonates and sulfates.
In hard water, the soap reacts with the calcium (which is relatively high in hard water) which is converted to "soap scum". When we are using hard water, more soap or detergent is required to get things clean, irrespective of if we need to wash our hands, hair, or our laundry.
Soft water is defined as the surface water which contains low concentrations of ions and in particular is low in ions of calcium and magnesium. Soft water occurs naturally in the areas where rainfall and the drainage basin of rivers are formed of hard, impervious and calcium-poor rocks.

Complete step by step answer:
We know that polysulphate is used to remove calcium sulphate and magnesium sulphate impurities from water.
Polyphosphates are usually used as water softening agents because these compounds form soluble complexes with cations which are responsible for the hardness of water.

\[2CaS{O_4} + {Na_2}[{Na_4}(P{O_3})6] \to {Na_2}[{Ca_2}({PO_3})6] + 2{Na_2}S{O_4}\]
\[2MgS{O_4} + {Na_2}[{Na_4}(P{O_3})6] \to {Na_2}[{Mg_2}({PO_3})6] + 2{Na_2}S{O_4}\]

So, the correct answer is Option C.

Note:
Polyphosphates are polymeric oxyanions and are defined as the salts or esters of tetrahedral \[P{O_4}\] (phosphate) structural units with shared oxygen atoms. Polyphosphates can form either linear or a cyclic ring structure. Phosphates is also an important component in various other products which are used in our daily life such as making baking powder and cola drinks, and great quantities are used in fertilizers and detergents.
A good water-softening system removes calcium and magnesium ions from hard water and thereby replaces it with sodium ions. The typical work of household soaps and detergents is interfered by calcium and magnesium ions but not by sodium. The water-softening process thus makes detergents more effective in removing dirt and oils from clothing and dishes. Such waters have percolated through limestone formations and thus, contain bicarbonate \[HC{O_3}^-\] along with small traces of carbonate \[C{O_3}^{2-}\] as the principal negative ions. We can accelerate the reaction by boiling the water and driving off the carbon dioxide gas.