Question

explain the formation of scum when hard water is treated with soap

Answer 1

Hardness in water is the calcium and magnesium dissolved in it. Soap has hydroxides or carbonates in it, they combine with the soap to form insoluble precipitates. And since there is usually dirt involved when you use soap, some of the dirt gets trapped and the precipitate looks scummy.

Answer 2

Soap does not work properly when the water is hard. A soap is a sodium or potassium salt of long chain carboxylic acids. Hard water contains salts of calcium and magnesium. When soap is added to hard water, calcium and magnesium ions, which are present in hard water, readily react with the carboxylate ion of the soap to form insoluble substance called scum. A lot of soap is wasted in the process.