Problem 37 - A Complete Resource Book in Chemistry for JEE Main

Question

Which of the following substance is not used in the preparation of lyophilic sol? (a) gum (b) gelatine (c) metal sulphide (d) starch

Step-by-Step Solution

Answer

(c) metal sulphide is not used in the preparation of lyophilic sol.

Step 1: Understand the Terms

First, let's understand the terms in the question. A lyophilic sol is a colloidal suspension where the dispersed phase has a strong affinity for the dispersion medium (often water). Lyophilic sols are also referred to as reversible sols because they can be re-dispersed in a medium after evaporation of the liquid.

Step 2: Examine Each Option

Now, examine each option to determine whether it can form a lyophilic sol. - Gum: This substance is known to form lyophilic sols readily due to its strong interaction with water. - Gelatine: Gelatine also forms lyophilic sols as it is water-soluble and can reversibly combine with water. - Metal sulphide: Metal sulphides typically form lyophobic sols, which do not easily dissolve in the dispersing medium. - Starch: Starch can form lyophilic sols because it swells and disperses when mixed with water.

Step 3: Identify the Outlier

From the examination, metal sulphides were identified as forming lyophobic sols. They do not form reversible colloids readily with water, hence are not used in creating lyophilic sols compared to the other substances listed.

Key Concepts

Understanding Colloidal Suspension

In chemistry, a colloidal suspension represents a state where tiny particles are dispersed throughout a different substance, like particles of oil in water. These particles are larger than molecules but small enough so that they don’t settle out quickly. Because they don’t settle, a colloidal suspension appears uniform and stable.
Colloids differ from true solutions in that the dispersed particles are big enough to scatter light (a phenomenon known as the Tyndall effect), but not big enough to be seen with just the naked eye.
Colloidal suspensions are crucial in many fields, including food, medicine, and industrial applications due to their unique properties like stability and homogeneity.

The Role of Dispersion Medium

The dispersion medium is the continuous phase in which the colloidal particles are distributed. It plays a vital role in the formation and stability of colloidal systems. For lyophilic sols, this medium is often water but can be other liquids too depending on the application.
The dispersion medium impacts how well the colloidal particles remain suspended.

• If the medium interacts favorably with the dispersed particles, it helps keep them suspended and stable.
• This interaction is crucial in lyophilic sols, which can be redispersed after evaporation because of this affinity or love for the medium.

In contrast, with lyophobic sols, this interaction is weak or nonexistent, prompting particles to eventually settle.

Exploring Reversible Sols

Reversible sols, also referred to as lyophilic sols, have fascinating properties. Because the dispersed particles have an affinity for the dispersion medium, they can easily redissolve or redisperse once in contact with the medium again.
This reversibility offers practical benefits:

• If a reversible sol dries out, you can usually just add the original medium back, and it will reform the sol without much difficulty.
• This ease of reformation is why gum, gelatine, and starch can form effective, reversible sols.

Reversible sols find applications in industries where these properties are advantageous, such as in food production and pharmaceuticals.

Understanding Lyophobic Sols

Lyophobic sols are the opposite of lyophilic sols. Their particles have little to no affinity for the dispersion medium, typically resulting in low stability. Common examples include metal sulphides and other inorganic salts.
Key points about lyophobic sols include:

• They don't easily redisperse once the medium is removed.
• The particles tend to aggregate, leading to settling or the formation of a sediment.
• This nature makes them generally irreversible, as opposed to lyophilic sols.

Due to their properties, lyophobic sols require stabilizing agents or energy input to remain as a colloid, challenging their use in everyday applications.