Question

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

Answer 1

Hint: Think about what type of ions and salts have to be present in water for it to be classified as hard water. Now, consider what kind of reactions may take place between soap and the ions present in hard water.

Complete step by step solution:
> Water is classified as hard water when the concentration of certain ions like $C{a}^{2+}$ and $M{g}^{2+}$ exceed a certain limit. Hard water has high mineral content, this is caused due to percolation of water through certain ores like gypsum, limestone, and chalk that are largely made up of calcium and magnesium carbonates, bicarbonates and sulphates.
> Soaps are basically sodium and potassium salts of long chain fatty acids; these salts help in the cleansing and emulsifying action that is required for cleaning purposes and formation of foam.
> When soap is dissolved in hard water, it reacts with the calcium and magnesium ions that are present there and form calcium and magnesium salt of fatty acids. These salts are not miscible in water at all and form a white layer on the surface of the water called scum. This scum prevents the foaming action and thus cleaning, inhibiting the function of soap.
The reactions that occur are:
$$RCO{O}^{-}N{a}^{+}+C{a}^{2+}\stackrel{}{\to }{(RCO{O}^{-})}_{2}Ca+2N{a}^{+}$$
$$RCO{O}^{-}N{a}^{+}+M{g}^{2+}\stackrel{}{\to }{(RCO{O}^{-})}_{2}Mg+2N{a}^{+}$$
> Thus, although hard water may be beneficial for the health if the concentration of ions present is in a reasonable proportion, it is preferred for hard water to be softened before being used to wash utensils and clothes.

Note: The scum (calcium and magnesium fatty acids) formed when soap reacts with hard water is not soluble in water at all. But when synthetic detergents are added to hard water, they still retain most of their cleansing action as instead of sodium salts of fatty acids like soaps, they have ammonium salts of long chain carboxylic acids. Remember that soap forms scum in hard water while detergent does not.