CBSE Class 10 Science 2017 · Paper 31/1/1 · Question 19

Difference between soaps and detergents: Soaps: sodium or potassium salts of long-chain carboxylic (fatty) acids, with the ionic end $-{\text{COO}}^{-}{\text{Na}}^{+}$ | Detergents: generally sodium salts of long-chain sulphonic acids, with the ionic end $-{\text{SO}}_3^{-}{\text{Na}}^{+}$ (or ammonium salts with chloride or bromide ions). Soaps: form an insoluble scum with hard water, so they do not work well in it | Detergents: form no scum and work well in both soft and hard water. Soaps: biodegradable | Detergents: many are not biodegradable. Cleansing action: a soap molecule has an ionic end that dissolves in water and a long hydrocarbon tail that dissolves in oil. When soap is applied to oily dirt, the tails of many soap molecules dissolve in the oil and the ionic ends stay in the water, so the dirt is enclosed in clusters called micelles. The micelles remain suspended in the water as an emulsion, and the dirt is washed away on rinsing (see the diagram). Hard water: it contains calcium and magnesium salts. Soap reacts with the ${\text{Ca}}^{2+}$ and ${\text{Mg}}^{2+}$ ions to form insoluble calcium and magnesium salts (scum), e.g. $2{\text{C}}_{17}{\text{H}}_{35}\text{COONa}+{\text{CaCl}}_2\to ({\text{C}}_{17}{\text{H}}_{35}\text{COO}{)}_2\text{Ca}\downarrow +2\text{NaCl}$. The soap is used up in forming scum, so lather does not form until these ions are removed, and much soap is wasted. Problems with detergents: (1) many detergents are not biodegradable, so they persist in rivers and lakes, cause foaming and harm aquatic life; (2) detergents containing phosphates cause excessive growth of algae in water bodies, which uses up the dissolved oxygen when the algae decay and kills fish.