As we know, soaps are cleaning agents. They are usually made by reacting alkali (eg. sodium hydroxide) with naturally occurring fat or fatty acids. Therefore we get salts of fatty acids.

Soaps have the general formula (RCO₂⁻)_nMⁿ⁺ (Where R is an alkyl, M is a metal and n is the charge of the cation). Hardness in water is caused due to the presence of soluble salts of magnesium and calcium in the form of chlorides or sulphates in water.

Lather is formed when the sodium or potassium salts dissolve in water. When soap is added to hard water, calcium and magnesium ions of hard water replaces so

dium or potassium ions in the soap and gets precipitated hence lather formation is hindered. Therefore, hard water does not form lather with soap.