Which of the following statement is not true for detergent molecule?

The correct answer is Option (4) → It is sodium salt of fatty acid

A sodium salt of a long-chain fatty acid (such as sodium stearate) is a soap. Synthetic detergents are not soaps: anionic detergents, for example, are sodium salts of sulphonated long-chain alcohols or hydrocarbons, as the extract shows.

• Option (1) It is not biodegradable — True for a detergent molecule. Detergents with a highly branched hydrocarbon chain are not degraded by bacteria and persist in rivers and ponds.

• Option (2) It is a surface active agent — True for a detergent molecule. Detergents lower the surface tension of water and collect at the surface.

• Option (3) It has a non polar organic part and a polar non organic part — True for a detergent molecule. The long hydrocarbon tail is non-polar and the –\( \ce{SO3^-Na+} \) head is polar.

• Option (4) It is sodium salt of fatty acid — Not true for a detergent molecule. This describes a soap.

Note: This question is from the pre-rationalised NCERT book (2022-23). The topic is not there in the latest NCERT book (2026-27), and it is also out of the CUET 2026 syllabus.

NCERT (Pre-rationalised edition, 2022-23):
Anionic Detergents – “Anionic detergents are sodium salts of sulphonated long chain alcohols or hydrocarbons. Alkyl hydrogensulphates formed by treating long chain alcohols with concentrated sulphuric acid are neutralised with alkali to form anionic detergents. Similarly alkyl benzene sulphonates are obtained by neutralising alkyl benzene sulphonic acids with alkali.”

Synthetic Detergents – “… Main problem that appears in the use of detergents is that if their hydrocarbon chain is highly branched, then bacteria cannot degrade this easily. Slow degradation of detergents leads to their accumulation. Effluents containing such detergents reach the rivers, ponds, etc. These persist in water even after sewage treatment and cause foaming in rivers, ponds and streams and their water gets polluted. These days the branching of the hydrocarbon chain is controlled and kept to the minimum. Unbranched chains can be biodegraded more easily and hence pollution is prevented.”