Animated Solution for Chemistry - s and p-Block Elements: What is reason of temporary hardness of water?

Have you ever tried washing your hands with soap, only to find that instead of a rich, bubbly lather, you get a sticky, white scum? If so, you have encountered the phenomenon of hard water.

Water hardness is primarily caused by the presence of dissolved multivalent metallic ions, most notably calcium (${\text{Ca}}^{2+}$) and magnesium (${\text{Mg}}^{2+}$). These ions interact with the molecules in soap, preventing them from doing their cleaning job effectively.

Hardness in water is not a single, uniform property. It is classified into two distinct categories based on how easily it can be removed: temporary hardness and permanent hardness.

Temporary hardness, as the name suggests, is relatively easy to deal with. It can be eliminated by simply boiling the water. Permanent hardness, on the other hand, requires chemical treatments like the addition of washing soda or the use of ion-exchange resins to soften the water.

So, what exactly causes this temporary hardness? It is caused by the presence of dissolved bicarbonates (also known as hydrogen carbonates) of calcium and magnesium.

The chemical formulas for these culprits are ${\text{Ca(HCO}}_3{)}_2$ and ${\text{Mg(HCO}}_3{)}_2$. When water flows over rocks containing limestone (${\text{CaCO}}_3$) or dolomite (${\text{CaMg(CO}}_3{)}_2$), it dissolves some of these minerals in the presence of dissolved carbon dioxide, forming these soluble bicarbonates.

Let's evaluate the options provided in our question to identify the true cause of temporary hardness.

Option (a) is ${\text{Na}}_2{\text{SO}}_4$ (Sodium sulfate). Sodium ions do not cause hardness because alkali metals do not form insoluble precipitates with soap.

Option (b) is ${\text{CaCl}}_2$ (Calcium chloride). While calcium does cause hardness, chlorides are responsible for permanent hardness, not temporary.

Option (c) is $\text{NaCl}$ (Sodium chloride). Again, sodium does not cause hardness.

Option (d) is ${\text{Ca(HCO}}_3{)}_2$ (Calcium bicarbonate). As we discussed, bicarbonates of calcium and magnesium are the exact cause of temporary hardness.

When we boil water containing temporary hardness, a fascinating chemical transformation occurs. The heat causes the soluble bicarbonates to decompose into insoluble carbonates.

This simple yet elegant reaction is why it is called 'temporary'. The insoluble calcium carbonate (${\text{CaCO}}_3$) precipitates out of the water (often forming the white scale you see in kettles) and can be easily filtered away, leaving behind soft water.

By understanding the chemical nature of dissolved minerals, we can easily deduce that temporary hardness is specifically linked to bicarbonates. Therefore, the correct reason for temporary hardness among the given choices is calcium bicarbonate.