Understanding the Tyndall Effect

The Tyndall effect is a phenomenon where light is scattered as a light beam passes through a colloidal dispersion. The individual suspension particles scatter and reflect light, making the beam visible. This effect is shown by colloidal solutions and fine suspensions, but not by true solutions.

For a mixture to show the Tyndall effect, the size of the dispersed particles must be comparable to the wavelength of light. True solutions have very small solute particles (typically < 1 nm in diameter) that are too small to scatter visible light effectively. Colloidal dispersions, on the other hand, have dispersed particles in the size range of approximately 1 nm to 1000 nm, which is large enough to scatter light.

Analyzing the Options and Particle Sizes

Let's examine each option based on the nature and size of the particles present:

• Aqueous solution of NaCl: When sodium chloride (NaCl) dissolves in water, it dissociates into sodium ions ($\text{Na}^+$) and chloride ions ($\text{Cl}^-$). These ions are very small, typically much less than 1 nm in size. This is a true solution. True solutions do not show the Tyndall effect.
• Aqueous solution of glucose: When glucose dissolves in water, it forms individual glucose molecules ($\text{C}_6\text{H}_{12}\text{O}_6$). Glucose molecules are also very small, on the order of 1 nm or less. This is a true solution. True solutions do not show the Tyndall effect.
• Aqueous solution of soap below critical micelle concentration: Soap consists of molecules with a long hydrocarbon chain and a polar head. Below the critical micelle concentration (CMC), soap molecules exist as individual ions or molecules dispersed in water. These individual particles are small, similar in size to solute particles in a true solution. Therefore, the solution behaves like a true solution and does not show the Tyndall effect.
• Aqueous solution of soap above critical micelle concentration: Above the critical micelle concentration (CMC), soap molecules aggregate together to form clusters called micelles. Micelles are formed when the concentration of soap is high enough that the individual molecules can arrange themselves into spherical structures, with the hydrophobic tails pointing inwards and the hydrophilic heads pointing outwards towards the water. Micelles are much larger than individual soap molecules or ions, typically ranging in size from a few nanometers to several tens of nanometers. These micellar aggregates are within the colloidal size range. Therefore, an aqueous solution of soap above the critical micelle concentration forms a colloidal solution and will show the Tyndall effect.

Conclusion

Based on the analysis of particle sizes and the nature of the mixtures, only the aqueous solution of soap above the critical micelle concentration contains particles large enough (micelles) to scatter light and exhibit the Tyndall effect. The other options are true solutions with particles too small to scatter light.

Revision Table: Key Concepts

Additional Information: Micelles and CMC

Soaps and detergents are examples of surfactants. Surfactant molecules have a dual nature: a hydrophilic (water-loving) part and a hydrophobic (water-hating) part. In water, below a certain concentration called the Critical Micelle Concentration (CMC), these molecules are dispersed individually.

Above the CMC, the hydrophobic tails of the molecules cluster together in the interior, avoiding contact with water, while the hydrophilic heads remain on the surface, facing the water. This structure is called a micelle. Micelles are dynamic structures, constantly forming and breaking, but their average size is in the colloidal range (typically 5-20 nm for many surfactants), making them capable of scattering light and exhibiting the Tyndall effect.

The CMC is a specific concentration for each surfactant in a given solvent at a particular temperature. For example, the CMC of sodium dodecyl sulfate (SDS), a common detergent, is around 8 millimoles per liter (mM) in water at 25°C.