Question

Colloidal solutions are not classified on the basis of : (a) molecular size (b) nature of the particles (c) surface tension value (d) interaction between disperse phase and dispersion medium

Short answer

Colloidal solutions are not classified on the basis of their surface tension value (c).

Step-by-step solution

Understanding colloidal solutions

A colloidal solution, or colloid, is a mixture where one substance of microscopically dispersed insoluble or soluble particles is suspended throughout another substance. The classification of colloids can be based on various factors such as the type of particles, size, and interactions between them, but not on properties that are not directly related to the dispersion of particles.

Identifying classification factors

The properties used to classify colloidal solutions include: (a) the molecular size of the particles in the colloidal range (approximately 1 to 1000 nanometers), (b) the nature of the particles (whether they are lyophilic or lyophobic), and (d) the type of interaction between the disperse phase (the colloidal particles) and the dispersion medium (the substance in which the particles are dispersed).

Eliminating the incorrect options

Since colloidal solutions can be classified based on molecular size, nature of the particles, and interaction with the dispersing medium, options (a), (b), and (d) are ways in which colloidal solutions can be classified. Thus, they can be eliminated.

Selecting the correct answer

Surface tension value is a property related to the interfacial tension between two phases (e.g., liquid and gas) and is not a classification basis for colloidal solutions. Therefore, option (c) 'surface tension value' is the correct answer as it is the property not used to classify colloidal solutions.

Key concepts

Molecular Size

The molecular size of particles in a colloidal solution is a fundamental characteristic that is crucial in its classification. This is because colloidal particles fall within a specific size range, measuring approximately 1 to 1000 nanometers (nm). Not too large to settle under gravity, and not too small to be considered true solutions, colloidal particles bridge the gap between fine suspensions and true solutions.

Understanding that colloids are composed of particles within this distinct size range helps students differentiate them from other types of mixtures. For instance, if a particle is larger than 1000 nm, the mixture is often a suspension, and particles will settle out over time. Conversely, if the particles are smaller than 1 nm, it is a true solution where the solute particles are completely dissolved in the solvent.

Nature of Colloidal Particles

The nature of colloidal particles is another important aspect used to classify colloids. This primarily involves the interaction of the particles with the solvent, described as either lyophilic (solvent-loving) or lyophobic (solvent-fearing).

Lyophilic colloids are those where the particles have a great affinity for the solvent and are thus easily solvated. They are typically more stable and can reform after precipitation. On the other hand, lyophobic colloids do not mix well with the solvent and require special methods to be prepared, as the particles have little to no affinity for the solvent. This distinction is key because it determines how a colloidal solution will behave under different conditions and how it can be manipulated or used in applications.

Interaction Between Disperse Phase and Dispersion Medium

Lastly, the type of interaction between the disperse phase and the dispersion medium plays a significant role in the properties and classification of colloidal solutions. This interaction dictates how the particles are distributed within the medium and affects the overall stability of the colloid.

The interaction can result in the formation of different types of colloids, such as hydrophilic or hydrophobic, depending on the affinity of the colloidal particles for the dispersion medium. For example, in water, hydrophilic colloids disperse easily due to strong interactions with the water molecules, whereas hydrophobic colloids tend to agglomerate due to weak interaction with water. This concept is vital for understanding how to prepare, stabilize, and use colloids in various scientific and industrial processes.