Which one of  the following will show Tyndall effect?

Aqueous solution of soap above critical micelle concentration

The correct answer is option 4. Aqueous solution of soap above critical micelle concentration.

The Tyndall effect involves the scattering of light by particles within a colloidal system. To understand why certain solutions show the Tyndall effect while others do not, it’s essential to differentiate between true solutions and colloidal solutions.

Key Concepts:

True Solutions:

In a true solution, such as those formed by dissolving common salts or sugars like \(NaCl\) or glucose in water, the solute particles are molecular or ionic and are dispersed at the atomic or molecular level. These particles are so small (less than 1 nanometer) that they do not scatter visible light. Thus, true solutions do not exhibit the Tyndall effect.

Colloidal Solutions:

In colloidal solutions, the solute particles are larger, ranging from 1 nanometer to 1 micrometer in size. Examples include emulsions, foams, and micellar solutions. The particles in colloidal solutions are large enough to scatter light, causing the Tyndall effect. This makes the path of a light beam visible when it passes through the colloidal solution.

Options Analysis:

1. Aqueous solution of \(NaCl\):

Sodium chloride dissolves completely in water, forming a true solution where Na⁺ and Cl⁻ ions are dispersed at the molecular level. The particle size is too small to scatter light, so it does not show the Tyndall effect.

2. Aqueous solution of glucose:

Glucose in water also forms a true solution with glucose molecules dispersed at the molecular level. These molecules are too small to scatter light, so this solution does not show the Tyndall effect.

3. Aqueous solution of soap below critical micelle concentration (CMC):

Below the CMC, soap molecules are present as individual molecules in the solution. These molecules are too small to scatter light, so the Tyndall effect is not observed in this case.

4. Aqueous solution of soap above critical micelle concentration (CMC):

Above the CMC, soap molecules aggregate to form micelles. Micelles are larger particles, typically ranging from 1 to 100 nanometers. These particles are large enough to scatter light, making the Tyndall effect observable. This is why the Tyndall effect is seen in soap solutions above the CMC.

Summary:

The Tyndall effect is observed when light is scattered by particles that are larger than those in a true solution. Therefore, among the options provided:

True solutions (like \(NaCl\) and glucose) do not exhibit the Tyndall effect.

Colloidal solutions (like soap solutions above the CMC) do exhibit the Tyndall effect due to the larger particle size (micelles) that scatters light.

Thus, the correct option is: 4. Aqueous solution of soap above critical micelle concentration.