Question

Polyphosphates like sodium hexametaphosphate (calgon) 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

-There are two types of hardness of water namely permanent and temporary hardness, both of which cannot be removed by the same methods.
-The temporary hardness can be removed by simple boiling, by the permanent hardness requires softening agents or other methods for removal of the hardness.

Complete step by step answer:
In order to understand the term water softening agents, we must first know the difference between soft and hard water.
The most commonly observed difference between soft and hard water is that hard water reacts with soap in order to form scum of solid soap, which is nothing but the white lather type substance which forms as a precipitate, due to presence of calcium and magnesium ions in the hard water, and soft water does not form scum of soap. The formation of soap scum on your skin is the main reason behind the fact that it is easier to rinse from your body while using soap, when you shower with hard water.
Hardness in water is produced due to the presence of magnesium ions, calcium ions or both. With increase in concentrations of magnesium ions, calcium ions or both, hardness of water also increases. The combined concentration of magnesium and calcium observed in a solution is frequently referred to as total hardness.
When hard water is exposed to a higher temperature or in other words when it is boiled in the presence of bicarbonate ${{(HC{{O}_{3}})}^{-}}$ or carbonate $C{{O}_{3}}^{2-}$ the magnesium and/or calcium ions react with the bicarbonate and carbonate ions in order to form solid magnesium carbonate, calcium carbonate, or both.
Temporary hardness is generally more complex, because concentration of it is a function of the concentration of carbonates in relation to their reaction with calcium in magnesium.
Permanent hardness is a type of hardness which simply cannot be removed by boiling of water.
Now if we consider the question, it says that the polyphosphates like sodium hexametaphosphate also called calgon, are water softening agents. The reason for this is, the hexametaphosphate forms water soluble complexes with the cations, that is, calcium and magnesium cations, which are present in the hard water. Since, the calcium and magnesium ions present in hard water are cations, hence the appropriate option would be option C.
Hence the correct answer would be option C.

Note: The hard water contains calcium and magnesium ions which could form water soluble complexes when a water softening agent is added to it.
-Since, the cations which were responsible for the hardness of water are now consumed by the agents, as a result of this, the water becomes soft.