CBSE Class 10 Science 2019 · Paper 31/4/1 · Question 16(b)

Difference: soaps are sodium or potassium salts of long-chain carboxylic (fatty) acids, while detergents are generally sodium salts of long-chain sulphonic acids (or ammonium salts). Soaps form scum with hard water and do not work well in it; detergents work well in both soft and hard water. Cleansing action: a soap molecule has an ionic end that dissolves in water and a long hydrocarbon tail that dissolves in oil. In soap solution the tails dissolve in the oily spot and the ionic ends face the water, so the molecules form micelles with the oily dirt trapped at the centre. On rubbing, the micelles are pulled off the fabric and, being held in the water by their ionic ends, are washed away with the dirt. Hard water contains calcium and magnesium ions, which react with soap to form an insoluble precipitate called scum, so a large amount of soap is wasted and little lather forms: $2{\text{C}}_{17}{\text{H}}_{35}\text{COONa}+{\text{Ca}}^{2+}⟶({\text{C}}_{17}{\text{H}}_{35}\text{COO}{)}_2\text{Ca}\downarrow +2{\text{Na}}^{+}$ The problem is resolved by using detergents, whose charged ends do not form insoluble precipitates with calcium and magnesium ions; the hardness can also be removed before washing, by boiling (temporary hardness) or with washing soda (permanent hardness).