The correct option is Micelles are large enough to scatter light.
Explanation
Soap solutions are classified as colloidal solutions (specifically associated colloids). In water, soap molecules aggregate to form structures called micelles. The appearance of the solution is determined by how light interacts with these suspended particles.
Detailed Analysis:
- Micelles are large enough to scatter light is Correct: Soap molecules consist of a hydrophilic head and a hydrophobic tail. In water, they cluster together to form micelles, with the hydrophobic tails shielded inside and hydrophilic heads facing the water. These micelles have dimensions within the colloidal range (1 nm to 1000 nm). Particles of this size are large enough to scatter a beam of visible light passing through the solution. This scattering of light, known as the Tyndall effect, makes the path of light visible and causes the solution to appear cloudy or turbid.
- Soap reacts with oxygen to form smoke is Incorrect: Soap does not undergo combustion or oxidation with oxygen to produce smoke while in an aqueous solution. Smoke is a specific type of colloid involving solid particles suspended in a gas.
- Soap absorbs all light in solution is Incorrect: If a solution absorbed all light, it would appear black. Soap solutions are translucent or opaque due to light scattering, not total absorption.
- Suspended dust causes opalescence is Incorrect: While dust can cause turbidity in any liquid, the characteristic cloudiness of a soap solution is an intrinsic property caused by the formation of micelles, not by external contaminants like dust.
Key Takeaway: The cloudiness of a soap solution is due to the Tyndall effect, where colloidal particles (micelles) scatter light because their size is comparable to the wavelength of visible light.