Which phenomenon occurs when an electric field is applied to a colloidal solution and electrophoresis is prevented?

(A) Reverse osmosis takes place

(B) Electroosmosis takes place

(C) Dispersion medium begins to move in an electric field

(D) Dispersion medium becomes stationary

Choose the correct answer from the options given below:

(B), (C) only

The correct answer is option 4. (B), (C) only.

Let us delve into the concepts of electroosmosis and how it relates to the behavior of a colloidal solution under the influence of an electric field.

Key Concepts

Colloidal Solutions: Colloidal solutions consist of fine particles (the colloid) dispersed in a continuous medium (the dispersion medium or solvent). These particles are typically in the range of 1 nm to 1 µm.

Electrophoresis: Electrophoresis is the movement of charged particles (colloidal particles) in a colloidal solution under the influence of an electric field. In a typical scenario, these particles migrate towards the electrode of opposite charge.

Electroosmosis: Electroosmosis, also known as electrokinetic flow, is the movement of the dispersion medium (the solvent) relative to the stationary colloidal particles when an electric field is applied. This occurs because the electric field exerts a force on the charged layers around the colloidal particles.

Detailed Explanation

Electrophoresis Prevention:

If electrophoresis is prevented, it means that the colloidal particles are not moving in response to the electric field. This could happen due to various reasons, such as: 

The particles are too large or too neutral to move significantly. The electric field strength is not sufficient to overcome the forces holding the particles in place.

Electroosmosis Mechanism:

Even if electrophoresis is prevented, the electric field still affects the dispersion medium. This is because the dispersion medium contains ions that form an electric double layer around the colloidal particles. When an electric field is applied, it affects the charged layers in the electric double layer surrounding the particles. This leads to the movement of the dispersion medium in the direction of the electric field.

Specifically, the movement occurs because the applied electric field induces a flow of the solvent in the direction of the field due to the interaction between the field and the charged species in the double layer.

Dispersion Medium Movement:

(C) Dispersion medium begins to move in an electric field: In electroosmosis, the solvent moves in response to the electric field, which creates a bulk flow of the dispersion medium. This movement is observable even when the colloidal particles themselves are not moving.

Why Other Options Are Incorrect:

Reverse Osmosis: Reverse osmosis involves solvent movement through a semipermeable membrane due to a pressure gradient, not an electric field.

Dispersion Medium Becoming Stationary: This is incorrect in the context of electroosmosis. If electroosmosis occurs, the dispersion medium will not remain stationary; instead, it will move under the influence of the electric field.

Summary

In summary, when an electric field is applied to a colloidal solution and electrophoresis is prevented, electroosmosis occurs. This phenomenon causes the **dispersion medium** (solvent) to move due to the interaction of the electric field with the electric double layer around the colloidal particles. Therefore, the correct options are:

(B) Electroosmosis takes place

(C) Dispersion medium begins to move in an electric field

Hence, option 4 (B), (C) only accurately describes the phenomena occurring under these conditions.