Animated Solution for Chemistry - Surface Chemistry: The correct option among the following is

This question is a beautiful conceptual tour through the fascinating world of surface chemistry, specifically focusing on the properties and applications of colloids. Let's break down each option to understand the underlying physical chemistry.

Let's start with option (a). Why are colloidal medicines so highly regarded in pharmacology? The secret lies in their geometry. When a bulk substance is broken down into colloidal-sized particles (ranging from $1\text{ nm}$ to $1000\text{ nm}$), its total surface area increases exponentially.

In the human body, the effectiveness of a medicine often depends on how well it can be absorbed and assimilated by our tissues. A larger surface area provides more contact points for the medicine to interact with biological fluids and cells, leading to faster and more efficient absorption. Therefore, colloidal medicines are more effective precisely because they have a large surface area, making the statement in option (a) incorrect.

Moving to option (b), we encounter Brownian motion—the continuous, random, zig-zag motion of colloidal particles. This motion arises due to the unequal and unbalanced collisions between the particles of the dispersed phase and the molecules of the dispersion medium.

Now, imagine trying to run through a swimming pool filled with water versus a pool filled with thick honey. The honey offers much more resistance. Similarly, the viscosity of the dispersion medium acts as a physical resistance to the movement of colloidal particles. If the viscosity is very high, the particles will face immense drag, and their Brownian motion will become slower, not faster. Hence, option (b) is fundamentally flawed.

Option (c) brings us to a classic household chemistry application: purifying muddy water using alum (phitkari). Muddy water is essentially a colloidal suspension where the mud particles carry a negative charge. Because they all carry the same charge, they repel each other and stay suspended rather than settling down.

When we add alum, which is a double salt with the formula $K_{2}S{O}_4\cdot A{l}_2(S{O}_4{)}_3\cdot 24H_{2}O$, it acts as a powerful electrolyte. It releases highly charged $A{l}^{3+}$ ions into the water. According to the Hardy-Schulze rule, these highly positive ions strongly attract the negatively charged mud particles, neutralizing their charge. Once neutralized, the mud particles clump together (coagulate) and settle at the bottom due to gravity. This leaves the water clear and fit for drinking. Thus, option (c) is incorrect.

Finally, we arrive at option (d). Lyophobic (liquid-hating) sols are relatively unstable and rely heavily on the presence of an electrical charge on their surface to prevent them from coagulating.

Electrophoresis is the phenomenon where these charged colloidal particles migrate under the influence of an applied electric field. If we apply a voltage, the negatively charged particles will march towards the positive electrode (anode), and positively charged particles will head towards the negative electrode (cathode).

When these particles touch the oppositely charged electrode, they surrender their charge. Without their protective charge, they can no longer repel each other. They aggregate, grow in size, and ultimately precipitate out of the suspension. This principle is famously used in the Cottrell precipitator to remove carbon dust from industrial factory smoke. Therefore, option (d) is a perfectly accurate scientific statement and is our correct answer.