Problem 135 - A Complete Resource Book in Chemistry for JEE Main

Question

Which of the following will show Tyndall effect? (a) Aqueous solution of soap above critical micelle concentration (b) Aqueous solution of soap below critical micelle concentration (c) Aqueous solution of sugar (d) Aqueous solution of sodium chloride

Step-by-Step Solution

Answer

Option (a), aqueous solution of soap above critical micelle concentration, shows the Tyndall effect.

Step 1: Understanding the Tyndall Effect

The Tyndall effect is the scattering of light by particles in a colloid or very fine suspension. This effect is only observed when the size of the particles is sufficient to scatter light, typically in the range of 1 nm to 1000 nm.

Step 2: Analyzing Each Option

We need to determine whether each option contains particles large enough to scatter light and display the Tyndall effect: - (a) Aqueous solution of soap above critical micelle concentration forms micelles, which are colloidal particles that scatter light. - (b) Aqueous solution of soap below critical micelle concentration will not form large enough particles to scatter light significantly. - (c) Aqueous solution of sugar is a true solution, where the sugar molecules are dissolved, not forming large enough particles for scattering. - (d) Aqueous solution of sodium chloride is also a true solution without scattering particles.

Step 3: Making an Informed Decision

Option (a) has particles (micelles) within the size range sufficient to exhibit the Tyndall effect. The rest are either true solutions or do not contain particles large enough to cause the effect.

Key Concepts

Understanding Colloids

In the world of chemistry, a colloid is a type of mixture where one substance is dispersed evenly throughout another. The particles in a colloid are large enough to scatter light, leading to effects like the Tyndall effect. This scattering occurs because the particles in a colloid are typically in the size range of 1 nanometer to 1000 nanometers.
What makes colloids fascinating is their ability to appear uniform and homogeneous, but still scatter light due to their particle size.

• These particles are neither small enough to be considered true solutions, like salt dissolved in water, nor large enough to settle out, like sand in water.
• Examples of colloids include milk, fog, and aerosol sprays, which all have particles suspended in a medium, causing the light scattering effect.

Understanding colloids and their properties helps in identifying when the Tyndall effect will be observed.

The Role of Micelles

Micelles are unique structures formed in water by surfactant molecules like soap. At higher concentrations, soap molecules align themselves in a spherical form, trapping any greasy or non-polar substances inside. This formation is what we refer to as a micelle.

The arrangement occurs because of the amphiphilic nature of surfactant molecules. They have two ends: one attracted to water (hydrophilic) and another repelled by water (hydrophobic).

• Above a certain concentration, these molecules will spontaneously come together, creating micelles.
• This phenomenon explains why soap effectively cleans oil and grease by encapsulating them within micelles.
• The size of micelles is just right for scattering light, making them visible under conditions that allow for the Tyndall effect.

Micelles are a perfect illustration of how molecular interactions can lead to significant macroscopic properties like light scattering.

Critical Micelle Concentration (CMC) Explained

The Critical Micelle Concentration (CMC) is a key concept in understanding how micelles form. It represents the concentration of surfactant molecules, like those in soap, needed for micelle formation to begin.
Until this concentration is reached, surfactant molecules are dispersed evenly in the solution without forming micelles.

• Below the CMC, the solution primarily acts as individual surfactant molecules.
• Above the CMC, these molecules aggregate to form micelles, capable of scattering light and exhibiting the Tyndall effect.
• The CMC value is crucial for both scientific studies and practical applications including detergency, pharmaceuticals, and cosmetics.

Understanding CMC allows us to predict when a solution will transition from not showing the Tyndall effect to exhibiting it, due to the formation of light-scattering micelles.