In colloidal solutions, Brownian motion of the dispersed particles is mainly caused by which of the following microscopic processes?

Introduction / Context:
This question tests your understanding of Brownian motion in colloids, a classic concept in physics and physical chemistry. Brownian motion refers to the continuous, random, zigzag motion of tiny particles suspended in a fluid. It provides strong evidence for the existence of molecules and their incessant thermal motion. In colloidal systems, Brownian motion contributes to stability by preventing particles from settling under gravity. Knowing the true microscopic cause of this motion helps you connect kinetic theory to observable macroscopic behaviour.

Given Data / Assumptions:

• We are dealing with a colloidal solution having a dispersed phase (colloidal particles) and a dispersion medium (fluid).
• Brownian motion is the erratic movement of the dispersed particles seen under a microscope.
• The options mention coagulation, flashes of light and molecular collisions.
• We assume normal temperatures where molecules have significant thermal energy.

Concept / Approach:
According to kinetic theory, molecules of a fluid are in constant random motion due to their thermal energy. In a colloid, the dispersed particles are much larger than individual molecules but are still small enough to be strongly affected by collisions. When molecules of the dispersion medium bombard the dispersed particles from all sides in an uneven way, the resulting net force on each particle fluctuates rapidly in magnitude and direction. This produces the observed irregular path known as Brownian motion. Coagulation and light scattering are related phenomena in colloids but do not directly cause this zigzag movement.

Step-by-Step Solution:
Step 1: Recall that Brownian motion is the random, continuous motion of small suspended particles observed under a microscope. Step 2: Recognise that this motion persists even in the absence of external stirring, so it must come from internal molecular activity. Step 3: According to kinetic theory, molecules of the dispersion medium are constantly moving and colliding due to thermal energy. Step 4: Each colloidal particle is bombarded unevenly by these molecules, causing constant changes in the net force on the particle. Step 5: This irregular bombardment makes the particle perform a zigzag path, which is observed as Brownian motion, confirming that molecular collisions in the dispersion medium are the cause.

Verification / Alternative check:
If the temperature is increased, molecules of the dispersion medium move faster, and Brownian motion becomes more vigorous. If the viscosity of the medium is increased, the motion slows down, which also matches the idea of molecular bombardment. These trends would not follow naturally from explanations based on coagulation or light flashes. Brownian motion was historically used as strong evidence for molecular theory when direct observation of molecules was not yet possible, further supporting the collision based explanation.

Why Other Options Are Wrong:
Coagulation of particles is a separate process where colloidal particles aggregate and settle; it does not create the characteristic continuous zigzag motion. Erratic flashes of light are related to scattering or Tyndall effect, where light is scattered by the particles; they are an optical effect and not the mechanical cause of motion. All of the above is incorrect because only molecular collisions of the dispersion medium with dispersed particles directly produce Brownian motion.

Common Pitfalls:
Some learners confuse Brownian motion with optical phenomena like the Tyndall effect or with instability processes like coagulation. Another misunderstanding is to think that gravity or external vibrations are responsible. To avoid these errors, remember that Brownian motion is fundamentally a kinetic phenomenon caused by molecular impacts. Whenever you see a question mentioning random motion of colloidal particles, link it directly to collisions by the surrounding fluid molecules.

Final Answer:
Brownian motion in colloids is mainly caused by molecules of the dispersion medium colliding with dispersed particles.