Answer the question on the basis of passage given below:
Adsorption is the phenomenon of attracting and retaining the molecules of a substance on the surface of a solid resulting into a higher concentration on the surface than in the bulk. The variation in the amount of gas adsorbed by the adsorbent with pressure at constant temperature is expressed as isotherm. A catalyst is a substance, when adsorbed enhances the rate of a chemical reaction without itself getting used up in the reaction. Depending on the type of the particles of the dispersed phase, colloids are classified as multimolecular, macromolecular and associated colloids and soap is such an example of associated colloids. Colloids show various properties such as Tyndall effect, colour, Brownian movement and charge development. There are positively charged sols and negatively charged sols. Due to this, electrophoresis and electroosmosis are shown by colloids.
The cleaning action of soap involves
A. Orientation of R group from \(RCOO^-\) is towards water.
B. CMC of 10−4 to 10−3 mol L−1
C. micelle formation
D. \(COO^-\) group from \(RCOO^-\) is hydrophilic.
Choose the correct answer from the options given below:
Answer & explanation
Correct answer: option 3
The correct answer is Option (3) → C and D only
A soap is the sodium or potassium salt of a long-chain fatty acid, \( \ce{RCOO^{-}Na^{+}} \). The \( \ce{RCOO^{-}} \) ion has a long hydrocarbon tail R, which is hydrophobic, and a polar COO− head, which is hydrophilic. Above the critical micelle concentration these ions gather into micelles: the tails dissolve in the grease and point inwards, while the heads face the water. The grease is held inside the micelle and is washed away with the water; this is the cleansing action of soap.
(A) Orientation of R group from \( \ce{RCOO^{-}} \) is towards water. — Incorrect. The hydrophobic R group points away from water, into the grease or the centre of the micelle; it is the \( \ce{COO^{-}} \) head that faces the water.
(B) CMC of 10−4 to 10−3 mol L−1 — Incorrect. The figure itself is right (the CMC of soaps is 10−4 to 10−3 mol L−1), but it is a property of soap solutions, the concentration above which micelles form; it is not a step of the cleaning action.
(C) micelle formation — Correct. The grease is trapped inside micelles, which is how it is removed.
(D) \( \ce{COO^{-}} \) group from \( \ce{RCOO^{-}} \) is hydrophilic. — Correct. The polar head stays in the water and keeps the micelle dispersed.
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):
Cleansing Action of Soaps – “… The cleansing action of soap is due to the fact that soap molecules form micelle around the oil droplet in such a way that hydrophobic part of the stearate ions is in the oil droplet and hydrophilic part projects out of the grease droplet like the bristles. Since the polar groups can interact with water, the oil droplet surrounded by stearate ions is now pulled in water and removed from the dirty surface.”