Problem 30 - A Complete Resource Book in Chemistry for JEE Main

Question

An example of an associated colloid is (a) vegetable oil (b) rubber latex (c) soap solution (d) milk

Step-by-Step Solution

Answer

The correct answer is (c) soap solution.

Step 1: Understanding Colloids

Colloids are mixtures where one substance is dispersed evenly throughout another. The particles of the dispersed substance are larger than those in a solution but smaller than those in a suspension. An associated colloid, specifically, is formed when a solute forms aggregates when its concentration in a solvent increases beyond a certain level.

Step 2: Identifying Types of Colloids

Associated colloids exist where molecules combine to form micelles at higher concentrations. In this context, common examples include soap solutions and surfactants. Vegetable oil is an emulsion colloid, rubber latex is a dispersion of polymer particles, and milk is also an emulsion colloid.

Step 3: Determining the Correct Answer

Among the options given, the soap solution is the one that forms micelles, making it an example of an associated colloid. The process of forming micelles is characteristic of associated colloids when certain compounds (often surfactants) are dissolved in a solvent.

Key Concepts

Micelles

Micelles are fascinating structures that form when molecules of surfactants, like soap, aggregate in a liquid. They occur when the concentration of surfactant molecules in a solution surpasses a certain threshold, known as the critical micelle concentration (CMC). Each surfactant molecule is composed of two distinct parts:

• A hydrophilic (water-attracting) ‘head’
• A hydrophobic (water-repelling) ‘tail’

When in water, the hydrophobic tails align inward to avoid water, while the hydrophilic heads face outward towards the surrounding water. This arrangement leads to the formation of spherical micelles. Micelles play a crucial role in cleaning processes by trapping oily substances inside their hydrophobic cores, allowing them to be washed away with water.

Colloid Solutions

Colloid solutions are mixtures where one substance is dispersed evenly throughout another. These mixtures are characterized by the size of the particles, which are larger than those found in normal molecules but smaller than those in suspensions. In colloid solutions:

• The dispersed phase is finely divided particles.
• The continuous phase is the substance in which particles are dispersed.
• The particle size typically ranges between 1 nm and 1000 nm.

Some easily visible examples of colloids are fog, milk, and jelly. A key characteristic of colloids is their stability—they do not settle out upon standing and cannot be separated by mere filtering. Associated colloids, like soap solutions, further illustrate this behavior by forming micelles at high concentrations.

Soap Solution

Soap solution is a common example of an associated colloid. When soap is dissolved in water above its critical micelle concentration, it forms micelles. Soap molecules are an effective cleansing agent due to their dual nature:

• The hydrophilic head attracts water.
• The hydrophobic tail draws in oils and grease.

This allows soap to trap dirt particles within micelles, after which they can be easily rinsed away. The soap solution, through micelle formation, highlights the cooperative behavior of molecules, where individual molecules group together to achieve a collective function. This property of soap solutions makes them indispensable in both household cleaning and numerous industrial applications.

Surfactants

Surfactants, or surface-active agents, are compounds that lower the surface tension between two substances, such as liquid and solid. By bridging the gap between oil and water, they enable the formation of micelles and play a crucial role as associated colloids:

• Reduce surface tension to promote mixing.
• Aid in emulsifying oils and dirt for easy cleaning.
• Used across industries including cleaners, textiles, and pharmaceuticals.

Due to their unique properties, surfactants are widely used in detergents, emulsifiers, and even in helping to disperse ingredients in medications. Their ability to form micelles makes them particularly valuable, as they enhance the solubility of hydrophobic compounds in water, making them applicable in various contexts where cleanliness and solubility are critical.