Question

Explain why soap from scum with hard water whereas detergent not

Answer 1

hard water containers calcium and magnesium salts. when soap is treated with hard water ,then the calcium and magnesium ions of hard water react with soap to form an insoluble precipitate called'scum'. the scum is formed because the calcium and magnesium salts of soaps are insoluble in water.

detergents can lather well even in hard water because they do not form insoluble calcium or magnesium salts on reacting with the calcium and magnesium ions present in hard water and hence are better cleansing agents.

hope it helps:)