Question

Why hard water do not form lather with soap.

Answer 1

Hard Water

• The water containing insoluble salts of calcium and magnesium is called hard water.
• It does not give lather with soap.

Because Hard water does not form lather with soap because of the precipitation of insoluble soap(calcium or magnesium stearate)

$\underset{\mathrm{calcium}\mathrm{chloride}\mathrm{in}\mathrm{hard}\mathrm{water}}{{\mathrm{CaCl}}_2\left(\mathrm{s}\right)}+\underset{\mathrm{Sodium}\mathrm{stearate}(\mathrm{soap}\mathrm{soluble})}{2{\mathrm{C}}_{17}{\mathrm{H}}_{35}{\mathrm{COO}}^{-}\hspace{0.17em}{\mathrm{Na}}^{+}\left(\mathrm{s}\right)}\to \underset{\mathrm{Calcium}\mathrm{stearate}(\mathrm{insoluble}\mathrm{soap})}{{\left({\mathrm{C}}_{17}{\mathrm{H}}_{35}\mathrm{COO}\right)}_2\mathrm{Ca}\downarrow \left(\mathrm{s}\right)}+\underset{\mathrm{sodium}\mathrm{chloride}}{\mathrm{NaCl}\left(\mathrm{s}\right)}$

Here calcium chloride in the hard water reacts with the sodium stearate of soluble soap giving a precipitate of calcium stearate of insoluble soap and sodium chloride. Instead of lather, a precipitate is formed.