Question

Explain the formation of scum when hard water is treated with soap.

Answer 1

Hard water often contains salts of calcium and magnesium. Soap molecules react with calcium and magnesium salts and form a precipitate. This precipitate begins floating as an off-white layer over water. This layer is called scum. Soaps lose their cleansing property in hard water because of formation of scum.

\[\ce{\underset{Hard water}{Ca^{2+}} + \underset{Soap}{2RCOONa} -> \underset{Cal. salt (ppt{.})}{(RCOO)2Ca} + 2Na+}\]

\[\ce{\underset{Hard water}{Mg^{2+}} + \underset{Soap}{2RCOONa} -> \underset{Mag. salt (ppt{.})}{(RCOO)2Mg} + 2Na+}\]

Answer 2

Scum is formed when a soap is used in hard water. This happens because a large amount of soap is utilised in reacting with the magnesium and calcium ions of hard water. An insoluble curdy precipitate, scum, sticks to the clothes making them difficult to be cleaned.