Question

give the reason of formation of scum when soaps are used with hard water​

Answer 1

The soap first reacts with calcium abd magnesium ions to form insoluble precipitate of calcium and magnesium salts of fatty acids which float as an off- white layer over wated. This layer is called scum. When a soap is used in hard water, a solid substance we call scum forms.

Answer 2

When a soap is used in hard water, a solid substance we call scum forms. This is because charged calcium and magnesium particles (called ions ) present in the water react with soap to form an insoluble substance. The new substance, scum, does not dissolve in water.