Question

Which phenomenon occurs when electric field is applied to a colloid solution and electrophoresis is prevented. (a) Electroosmosis takes place (b) Reverse osmosis takes place (c) Dispersion medium becomes stationary (d) Dispersion medium begins to move

Short answer

(a) Electroosmosis takes place.

Step-by-step solution

Understand the Definition of Colloid

A colloid is a heterogeneous mixture where one substance is dispersed in another in the form of small particles. The dispersion medium is the fluid in which the colloidal particles are suspended.

Introduction to Electrophoresis

Electrophoresis is the movement of colloidal particles towards the electrodes under the influence of an electric field. This results in the migration of the dispersed phase, i.e., the colloidal particles, rather than the dispersion medium.

Explanation of Electroosmosis

Electroosmosis occurs when an electric field causes the movement of the fluid that forms the dispersion medium. If the movement of the dispersed phase (electrophoresis) is restricted, the dispersion medium starts to move instead due to electroosmotic force.

Analysis of the Options

(a) Electroosmosis takes place - Occurs when the dispersion medium moves. (b) Reverse osmosis takes place - A process that desalines water, not related to colloid movement. (c) Dispersion medium becomes stationary - Contradicts the condition, as ideally, the medium moves. (d) Dispersion medium begins to move - Coincides with electroosmosis.

Choose the Correct Answer

Given that electrophoresis is prevented, the effect of the electric field causes the dispersion medium to move, not the colloidal particles themselves. Thus, electroosmosis takes place.

Key concepts

Colloid

A colloid is a unique type of mixture where small particles of one substance are scattered throughout another. Imagine a bowl of jelly, where the gel is the continuous phase and the water trapped inside is the dispersed phase. The dispersed particles in a colloid are generally between 1 nanometer and 1 micrometer in size. This small size keeps the particles suspended within the dispersion medium without settling out.
Colloids exhibit interesting properties, for instance, they can scatter light, which is known as the Tyndall effect. This scattering effect is why you can see a beam of sunlight in a dusty room. Importantly, the interaction between dispersed particles and the surrounding dispersion medium determines how the whole colloidal system behaves. When an electric field is applied, it can influence these interactions and lead to phenomena like electrophoresis or electroosmosis.

Electroosmosis

Electroosmosis is a fascinating mechanism that unfolds primarily in colloidal systems. When an electric field is applied to a colloidal solution, it can drive the flow of the dispersion medium rather than just the particles themselves. This happens when the movement of the dispersed particles is somehow restricted.
Here’s how it works:

• The dispersion medium (usually a liquid) carries ions that respond to the electric field.
• As these ions move, they exert a dragging force on the surrounding fluid.
• This results in the bulk movement of the medium through capillary or porous membranes, essentially causing a flow of the whole medium.

Electroosmosis is widely used in various applications, such as in the cleaning of soils or the movement of fluids in microfluidic devices, owing to its ability to precisely control fluid motion without the need for mechanical parts.

Dispersion medium

The dispersion medium is an integral part of any colloid. It serves as the host fluid in which tiny particles are spread out. The nature of this medium can be solid, liquid, or gas, but in many colloidal systems, it is often a liquid.
Alterations in the properties or movement of the dispersion medium can significantly influence the characteristics and behavior of the colloid. For example:

• A change in temperature can impact the viscosity of the dispersion medium, affecting how easily particles can move through it.
• Applying an electric field may cause the medium to shift or flow, as seen in electroosmosis.

Understanding the role of the dispersion medium helps in predicting and manipulating the behavior of colloids in various industrial and scientific applications, including paints, foods, and medical technologies. The interplay between the medium and dispersed particles is a key factor in the study of colloidal systems.