Why is common salt (sodium chloride) added during the preparation of soap?

Soaps are prepared by treating the esters of fatty acid with alkali. Sometimes common salt is also added to it. Why is common salt added during this process?

Have you ever observed while cleaning cloth that foam is formed with difficulty and an insoluble substance (scum) remains after washing with water? This is caused by the reaction of soap with the calcium and magnesium salts, which cause the hardness of water. Hence, you need to use a larger amount of soap. This problem is overcome by using another class of compounds called detergents as cleansing agents. Detergents are generally ammonium or sulphonate salts of long chain carboxylic acids. The charged ends of these compounds do not form insoluble precipitates with the calcium and magnesium ions in hard water. Thus, they remain effective in hard water. Detergents are usually used to make shampoos and products for cleaning clothes.

(i) How will you test the hardness of water?

(ii) Why soap forms scum (insoluble substance) with hard water?

(iii) How one can overcome the problem of scum formation? What values do you identify in this context?