CBSE Class 10 Science 2020 · Paper 31/4/3 · Question 17

(a) A soap is the sodium or potassium salt of a long-chain carboxylic acid, with a carboxylate end ($-{\text{COO}}^{-}{\text{Na}}^{+}$), e.g. sodium stearate. A detergent is an ammonium or sulphonate salt with a long hydrocarbon chain, e.g. the sodium salt of a long-chain sulphonic acid ($-{\text{SO}}_3^{-}{\text{Na}}^{+}$ end); it has no carboxylate group. (b) Take about 10 mL of distilled (soft) water in test tubes A and B and 10 mL of hard water in test tubes C and D. Add five drops of soap solution to A and C and five drops of detergent solution to B and D, and shake all four for the same time. Soap forms plenty of lather in soft water but little lather and a white curdy scum in hard water; detergent forms plenty of lather in both. Hence a detergent works well with all types of water, while a soap does not.