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 10⁴ to 10³ mol $L^{-1}$
C. micelle formation
D. $COO^-$ group from $RCOO^-$ is hydrophilic.

Choose the correct answer from the options given below:

C and D only

The correct answer is Option (3) → C and D only.

Let us break down the cleaning action of soap in detail, focusing on micelle formation and the properties of the hydrophilic group

Structure of Soap
Soaps are usually salts of fatty acids, which have a long hydrocarbon chain (hydrophobic tail) and a polar carboxylate group (hydrophilic head). The general formula for soap can be represented as \(RCOO^−Na^+\), where:

\(R\) is a long hydrocarbon chain (the hydrophobic tail).

\(COO^−\) is the carboxylate group, which is hydrophilic.

Micelle Formation

When soap is added to water:

Hydrophilic Heads: The polar carboxylate groups \((–COO^−)\) interact favorably with water molecules due to hydrogen bonding and dipole interactions. This hydrophilic nature helps the soap molecules dissolve in water.

Hydrophobic Tails: The long hydrocarbon tails, however, are non-polar and do not interact well with water. Instead, they tend to avoid water and aggregate together.

As a result of these opposing interactions, soap molecules arrange themselves into spherical structures called micelles. In a micelle:

The hydrophilic heads face outward toward the water, while the hydrophobic tails are tucked inside, away from the water.

This arrangement allows soap to effectively encapsulate oil and grease, which are hydrophobic, within the interior of the micelles

Cleaning Mechanism

Encapsulation of Dirt: When soap is applied to a surface (like skin or fabric), the micelles form around the dirt and grease. The hydrophobic tails interact with the oil, while the hydrophilic heads remain in contact with the water.

Washing Away Dirt: When rinsed with water, the micelles, now containing the trapped dirt and grease, are washed away, leaving the surface clean.

4. Properties of the Carboxylate Group

Hydrophilic Nature: The carboxylate group \((–COO^−)\) is polar and interacts well with water molecules. This hydrophilic property is crucial because it allows soap to dissolve in water and form the necessary micelles for effective cleaning.

Surface Activity: The hydrophilic part of soap helps reduce the surface tension of water, allowing it to spread and penetrate more effectively into fabrics or onto surfaces, aiding in the removal of dirt

Options Analysis

Option A (Orientation of R group towards water): This is incorrect. The hydrophobic R group actually orients away from water, toward other hydrophobic substances (like grease).

Option B (CMC of 10⁴ to 10³ mol L⁻¹): The critical micelle concentration (CMC) can vary widely based on the type of soap and conditions, and is typically much lower than this range for effective cleaning.

Option C (Micelle formation): This is correct and describes the essential process by which soap cleans.

Option D (\(COO^−\) group is hydrophilic): This is correct as it describes the polar nature of the carboxylate group.

Conclusio

The cleaning action of soap primarily relies on its ability to form micelles, encapsulating dirt and grease through the interactions between its hydrophilic and hydrophobic parts. Options C and D correctly reflect these aspects of soap's functionality in cleaning.