Give a chemical reaction to show why not to use ordinary soap

Dear Student,

The question seems to be incomplete. It should be why not to use ordinary soap in hard water. 
Ordinary soap cannot be used in hard water as it will lead to the formation of an insoluble substance called scum. The Ca²⁺ or Mg²⁺ ions in hard water displace Na⁺ or K⁺ from the soap and form scum. The reaction is shown below:

$$\begin{gathered} 2{\mathrm{C}}_{17}{\mathrm{H}}_{35}\mathrm{COONa} +\hspace{0.17em} {\mathrm{MgCl}}_2 \to ({\mathrm{C}}_{17}{\mathrm{H}}_{35}\mathrm{COO}{)}_2\mathrm{Mg} + \mathrm{NaCl} \\ \mathrm{Soap} \mathrm{Scum} \\ 2{\mathrm{C}}_{17}{\mathrm{H}}_{35}\mathrm{COONa} +\hspace{0.17em} {\mathrm{CaCl}}_2 \to ({\mathrm{C}}_{17}{\mathrm{H}}_{35}\mathrm{COO}{)}_2\mathrm{Ca} + \mathrm{NaCl} \\ \mathrm{Soap} \mathrm{Scum} \end{gathered}$$