Hard water contains ion of

Hard water does not produce lather with soap is called hard water. Hardness is due to the presence of bicarbonate, chlorides and sulphates of $\mathrm{Ca}$ and $\mathrm{Mg}$. $\begin{aligned} & \mathrm{M}^{2+}+2 \mathrm{C}_{17} \mathrm{H}_{35} \mathrm{COONa} \longrightarrow \underset{\text { Sodium stearate (soap) }}{\left(\mathrm{C}_{17} \mathrm{H}_{35} \mathrm{COO}\right)_2 \mathrm{M}+2 \mathrm{Na}^{+}} \\ & \left(\mathrm{M}=\mathrm{Ca}^{2+}, \mathrm{Mg}^{2+}\right) \\ & \mathrm{Ca}^{2+} \text { or } \mathrm{Mg}^{2+} \text { ion present in hard water react with } \\ & \text { soap to form precipitate of } \mathrm{Ca} \text { and } \mathrm{Mg} \text { salt of fatty } \end{aligned}$ acids and hence, no lather is produced.