Question

Electro-osmosis is observed when: (a) dispersion medium particles begins to move in an electric field (b) dispersed phase begins to move in an electric field (c) both (a) and (b) (d) none of the above

Short answer

The phenomenon of electro-osmosis is observed when the dispersion medium particles begin to move in an electric field, which is option (a).

Step-by-step solution

Understanding Electro-osmosis

Electro-osmosis is a phenomenon in colloids where the dispersion medium (typically a liquid) moves relative to a fixed surface under the influence of an electric field. It is due to the movement of the liquid containing the ions.

Analyzing the Options

Option (a) describes the movement of dispersion medium particles under the effect of an electric field, which is consistent with the definition of electro-osmosis. Option (b) describes the movement of dispersed phase, which is not electro-osmosis but electrophoresis. Options (c) and (d) therefore cannot be correct as they either combine incorrect concepts or deny the phenomenon.

Concluding the Correct Answer

Since electro-osmosis involves the movement of the dispersion medium due to an electric field, the correct answer must describe this process, which is option (a).

Key concepts

Colloids

Colloids are heterogeneous mixtures in which one substance is dispersed evenly throughout another. The dispersed substance, known as the dispersed phase, is made up of tiny particles or droplets that are mixed within a continuous substance called the dispersion medium. Unlike solutions, colloidal particles do not settle out upon standing and are too small to be seen by the naked eye. However, they are large enough to scatter light, a phenomenon known as the Tyndall effect.
Colloids play a vital role in various fields, including medicine, food, and industrial applications. Some everyday examples of colloids include milk (where fat globules are dispersed in water), jelly, and smoke (solid particles in gas). Understanding colloids helps in refining methods for their stabilization, which is critical for products like emulsions and foams.

Electric Field

An electric field is a region around a charged particle or object within which a force would be exerted on other charged particles or objects. It is a vector field, which means it has both magnitude and direction. The strength of the electric field at a particular point is defined as the force per unit charge that a positive test charge would experience at that location.
Electric fields are fundamental to numerous natural phenomena and the functioning of a variety of technologies. For example, they are essential in the operation of capacitors in electronic circuits. Visualizing electric fields can be done with field lines: lines that point in the direction a positive test charge would move if placed in the field. Closer lines indicate a stronger field.

Dispersion Medium

The dispersion medium in colloids is the continuous phase that contains the dispersed particles. It is essentially the 'solvent' of the colloid. The dispersion medium can be a solid, liquid, or gas, but in many common colloids, it is a liquid.
The characteristics of the dispersion medium, such as its polarity, viscosity, and dielectric constant, can influence the stability and properties of a colloid. For instance, in an aqueous colloid (where water is the dispersion medium), the polarity of water is significant for stabilizing charged colloidal particles. The interaction between the dispersion medium and dispersed phase is also important in understanding phenomena like electro-osmosis.

Electrophoresis

Electrophoresis is a technique used to separate charged particles under the influence of an electric field. It is different from electro-osmosis; while electro-osmosis involves the movement of the dispersion medium relative to a stationary surface, electrophoresis describes the movement of the dispersed phase, often for the purpose of analysis or purification.
In a laboratory setting, electrophoresis is commonly carried out using gels or liquids as mediums. DNA, RNA, and proteins are typical molecules that can be separated using electrophoresis based on their size and charge. By applying an electric field, these molecules migrate at different rates, allowing them to be separated and studied. This process is fundamental in molecular biology research and forensics.