Question

Does Boiling Water Reduce Hardness?

Answer 1

Yes, it does reduces the hardness of water temporarily. But heating only works if Bicarbonate salts of $\mathrm{Ca}$ and $\mathrm{Mg}$ are present in water (Temporary Hardness). Hardness in water occurs due to the presence of ${\mathrm{Ca}}^{2+}$ and ${\mathrm{Mg}}^{2+}$ions.

Bicarbonates of $\mathrm{Ca}$ and $\mathrm{Mg}$ are highly soluble in water. And when Bicarbonate salts of $\mathrm{Ca}$ and $\mathrm{Mg}$ are heated they decompose and give out their respective carbonates and release water and carbon dioxide. These carbonates are insoluble in water hence reducing the concentration of $\mathrm{Ca}$ and $\mathrm{Mg}$ ions in water and thus reducing its hardness.

$$\begin{gathered} \mathrm{Ca}{\left({\mathrm{HCO}}_3\right)}_2\stackrel{\mathrm{Heat}}{\to }{\mathrm{CaCO}}_3+{\mathrm{H}}_2\mathrm{O}+{\mathrm{CO}}_2 \\ \mathrm{Ca}{\left({\mathrm{HCO}}_3\right)}_2-\mathrm{Soluble}\mathrm{in}\mathrm{water} \\ {\mathrm{CaCO}}_3-\mathrm{Insoluble}\mathrm{in}\mathrm{water} \end{gathered}$$