$\begin{array}{1 1} \text{(a) (i) When soaps are dissolved in soft water, these ions displace calcium or potassium from their salts and form soluble calcium or magnesium salts of fatty acids. These insoluble salts separate as scum.(ii) Calcium and magnesium salts of detergents are soluble in water, so they easily form lather with hard water.} \\ \text{(b) (i) When soaps are dissolved in hard water, these ions displace sodium or potassium from their salts and form insoluble calcium or magnesium salts of fatty acids. These insoluble salts separate as scum.(ii) Calcium and magnesium salts of detergents are soluble in water, so they easily form lather with hard water.} \\ \text{(c) (i) When soaps are dissolved in hard water, these ions displace sodium or potassium from their salts and form insoluble calcium or magnesium salts of fatty acids. These insoluble salts separate as scum.(ii) Calcium and sulphur salts of detergents are insoluble in water, so they easily form lather with hard water.}\\(d)\;\text{None of these} \end{array} $
