Question

Why does hard water not form lather with soap?

Answer 1

Hard water contains $C{a}^{2+}$ and $M{g}^{2+}$ ions while soaps contain sodium salts of fatty acids.

Thus, when soap is added to hard water, the $C{a}^{2+}$ and $M{g}^{2+}$ ions present in hard water react with soap and sodium salts are converted to their respective calcium and magnesium salts which are precipitated as scum.

When the scum dries on washed clothes, it makes them stiff and no lather formation takes place.

Therefore, when hard water combines with soap solution, a white curdy precipitate called scum is formed due to which it does not form lather with soap.