Which statement is not true for a detergent molecule?

The correct option is "It is sodium salt of fatty acid".

Let us understand why this statement is not true for a detergent molecule by breaking down the differences between soaps and synthetic detergents:
1. Chemical Composition of Soaps: Soaps are sodium or potassium salts of long-chain fatty acids (such as stearic acid, palmitic acid, or oleic acid). Therefore, the statement "It is sodium salt of fatty acid" specifically describes a soap, not a detergent.
2. Chemical Composition of Detergents: Synthetic detergents are sodium salts of long-chain alkyl hydrogen sulfates or sodium salts of long-chain alkyl benzene sulfonic acids. They do not contain carboxylate ions derived from fatty acids.
3. Biodegradability: Many synthetic detergents (especially those with branched hydrocarbon chains) are non-biodegradable and can cause environmental pollution, making the statement "It is not biodegradable" generally applicable to many traditional synthetic detergents.
4. Surface Activity: Both soaps and detergents are surface-active agents (surfactants) because they reduce the surface tension of water, allowing it to wet surfaces more effectively.
5. Molecular Structure: A detergent molecule consists of a non-polar organic part (a long hydrocarbon chain that is lipophilic/hydrophobic) and a polar inorganic part (such as a sulfate ${\mathrm{―OSO}}^3{―}^{\mathrm{Na}}$ or sulfonate group ${\mathrm{―SO}}^3{―}^{\mathrm{Na}}$ which is hydrophilic).

Therefore, because a detergent is not a sodium salt of a fatty acid (which is the definition of a soap), this statement is not true for a detergent molecule.