Animated Solution for Chemistry - s and p-Block Elements: The temporary hardness of a water sample is due to compound $X$. Boiling this sample converts $X$ to compound $Y$. $X$ and $Y$, respectively, are

In the realm of water chemistry, hardness is primarily caused by the presence of dissolved calcium (${\text{Ca}}^{2+}$) and magnesium (${\text{Mg}}^{2+}$) ions.

When we talk specifically about temporary hardness, we are referring to the presence of bicarbonate ions (${\text{HCO}}_3^{-}$) paired with these metals. Thus, the compound $X$ responsible for this temporary hardness must be either magnesium bicarbonate, ${\text{Mg(HCO}}_3{)}_2$, or calcium bicarbonate, ${\text{Ca(HCO}}_3{)}_2$.

Boiling Hard Water The beauty of temporary hardness is that it can be easily removed by a simple physical process: boiling. When we heat the water, the thermal energy causes the soluble bicarbonates to decompose into insoluble precipitates, which can then be filtered out.

Let's look at the chemical reactions that occur during boiling. For calcium bicarbonate, the reaction is straightforward:

Here, calcium carbonate (${\text{CaCO}}_3$) precipitates out.

However, magnesium bicarbonate behaves differently, and this is where many students make a silly mistake! You might expect it to form magnesium carbonate (${\text{MgCO}}_3$), but instead, it forms magnesium hydroxide:

Why does magnesium form a hydroxide while calcium forms a carbonate? This is a classic JEE concept rooted in the solubility product ($K_{sp}$).

In aqueous solutions, the precipitate that forms is the one that is least soluble. For magnesium, the solubility product of magnesium hydroxide, ${\text{Mg(OH)}}_2$, is significantly lower than that of magnesium carbonate, ${\text{MgCO}}_3$. Therefore, ${\text{Mg(OH)}}_2$ precipitates preferentially.

Based on the options provided and our chemical analysis, if $X$ is ${\text{Mg(HCO}}_3{)}_2$, then $Y$ must be ${\text{Mg(OH)}}_2$.

This perfectly aligns with the first option, making it the correct answer. Always remember to watch out for the magnesium exception when dealing with water hardness!