Question

(a) What is a soap? Name one soap.
(b) Describe the structure of a soap molecule with the help of a diagram.
(c) Explain the cleansing action of soap. Draw diagrams to illustrate your answer.
​(d) Give the main differences between soaps and detergents.

Answer 1

(a) Soaps are sodium and potassium salts of long chain fatty acids containing 12-18 carbon atoms. For eg. sodium salt of stearic acid is sodium stearate (${\mathrm{C}}_{17}{\mathrm{H}}_{35}\mathrm{COONa}$).

(b) A soap is a sodium or potassium salt of long-chain fatty acids. It has one polar end (the end with the sodium or potassium ion) and one non-polar end (the end with the fatty-acid chain). The polar end is hydrophilic in nature, i.e., this end is attracted towards water. The non-polar end is hydrophobic, but lipophilic in nature, i.e., it is attracted towards hydrocarbons, but not attracted towards water.




(c)

Each soap molecule has two ends. These are:
(i) The polar, Ionic hydrophilic end.
(ii) The non-polar, non-ionic hydrophobic end.
Mechanism of cleansing action of soap:
1. Soap gets dissolved in water
2. The non-polar end gets directed towards the center and the polar end gets directed outwards.
3. A cluster compromising of this arrangement is formed which is known as a micelle.
4. The dirty cloth is put in a soap solution.
5. Dust or oil is hydrophobic in nature. The non-polar end gets attached to the oil or dirt present in the cloth and polar end remain attached to water.
6. The dirty cloth is agitated in the solution.
7. Oily and greasy particles entrapped in soap micelle gets dispersed in water.
8. Soap water gets dirty and the cloth gets cleaned.

(d)

Soaps	Detergents
1. Soaps are the sodium salts of long chain carboxylic acids.	1. Detergents are sodium salts of long-chain benzene sulphonic acids.
2. The ionic group is soaps is COO-Na+.	2. The ionic group in soaps is ${\mathrm{SO}}_3^{-}{\mathrm{Na}}^{+}\mathrm{or}{\mathrm{SO}}_4^{-}{\mathrm{Na}}^{+}$
3. Soaps are not useful when water is hard.	3. Detergents can be used for washing purpose even when water is hard.
4. Soaps are biodegradable.	4. Some detergents are non-biodegradable.
5. Soaps have relatively weak cleansing action.	5. Detergents have a strong cleansing action.