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Soap is the sodium salt of stearic acid whose chemical formula is C₁₇H₃₅COONa. It is a long chain of fatty acids and in the presence of the ordinary water (no hardness), it forms stearic acid which is responsible for the lather formation in water.

$\begin{array}{ccc}\underset{\mathrm{Sodium} \mathrm{Stearate} \left(\mathrm{Soap}\right)}{{\mathrm{C}}_{17}{\mathrm{H}}_{35}\mathrm{COONa}}+{\mathrm{H}}_2\mathrm{O}& \to & \underset{\mathrm{Stearic} \mathrm{acid}}{{\mathrm{C}}_{17}{\mathrm{H}}_{35}\mathrm{COOH}}+\mathrm{NaCl}\\ {\mathrm{C}}_{17}{\mathrm{H}}_{35}\mathrm{COOH}+{\mathrm{C}}_{17}{\mathrm{H}}_{35}\mathrm{COONa}& \to & \mathrm{Lather}\end{array}$

In the hard water (having Ca²⁺ or Mg²⁺), there is a formation of scum or insoluble precipitate which retards the lather formation in water.

${\mathrm{Ca}}_{\left(\mathrm{aq}\right)}^{2+}+2{\mathrm{C}}_{17}{\mathrm{H}}_{35}\mathrm{COONa}\to \underset{\mathrm{Insoluble} \mathrm{precipitate}}{\left({\mathrm{C}}_{17}{\mathrm{H}}_{35}{\mathrm{COO}}_2{)}_2\mathrm{Ca}\downarrow \right.+2{\mathrm{Na}}^{+}}$