Problem 149 - A Complete Resource Book in Chemistry for JEE Main

Question

Which among the following are lyophilic in nature? (a) Gold (b) Gum (c) Starch (d) Gelatin

Step-by-Step Solution

Answer

Gum, Starch, and Gelatin are lyophilic in nature; Gold is not.

Step 1: Understand Lyophilic Colloids

Lyophilic colloids are those which have a strong affinity between the dispersed phase and the dispersion medium. This affinity allows the colloid to form a stable solution or gel with the dispersion medium. The particles are well solvated and can be easily dissolved or dispersed in the medium.

Step 2: Identify Characteristics of Options

Let's examine each option: (a) Gold - It is actually a lyophobic colloid with poor affinity towards solvents. (b) Gum - Known for forming gels by absorbing water, indicating strong solvent affinity. (c) Starch - Naturally disperses in water to form a stable paste or gel, indicating solvent affinity. (d) Gelatin - Can easily dissolve in water to give a gel, showing strong affinity towards the solvent.

Step 3: Select Lyophilic Compounds

Based on the characteristics: - Gum (b) is lyophilic. - Starch (c) is lyophilic. - Gelatin (d) is lyophilic. Gold (a), however, does not exhibit lyophilic properties as it does not easily dissolve or disperse in a solvent.

Key Concepts

Colloidal Chemistry

Colloidal chemistry studies the properties and behaviors of mixtures where microscopic particles are evenly distributed throughout another substance. These substances, known as colloids, are unique because their particles are not dissolved but remain suspended within the dispersing medium. This creates a mixture that exhibits distinct behaviors compared to solutions. Colloidal systems can be found in everyday products like milk, jelly, and even paint.

• They can be classified into types based on the interaction between the dispersed particles and the dispersing medium.
• The size of colloidal particles ranges from 1 to 1000 nanometers, striking a balance between true solutions and suspensions.
• Colloids have properties like the Tyndall effect, which scatters light, and Brownian motion, which involves the random movement of particles.

Colloidal chemistry is fundamental for understanding many natural and industrial processes.

Lyophobic vs Lyophilic

Colloids are categorized based on their affinity towards the dispersion medium into lyophobic and lyophilic colloids. The term 'lyophobic' means solvent-repelling, whereas 'lyophilic' means solvent-attracting.

• **Lyophilic colloids** have particles that exhibit a strong attraction to their solvent. This makes them easy to dissolve or disperse, forming stable mixtures.
• **Lyophobic colloids**, on the other hand, have little to no affinity towards their solvent, making them inherently unstable and prone to aggregation without the use of stabilizers.

Understanding these distinctions helps in predicting the behavior of colloids in different environments. For example, gum, starch, and gelatin are lyophilic because they dissolve well in water, while gold, being lyophobic, tends to separate out unless stabilized.

Solvent Affinity in Colloids

In colloidal systems, the affinity of the colloidal particles towards the solvent plays a huge role in determining the system's stability. Solvent affinity is the tendency of the colloidal particles to interact with the solvent molecules.

• High affinity results in the spontaneous formation of stable colloids. Particles are surrounded by a layer of solvent molecules, known as solvation or hydration in water.
• Low affinity may lead to unstable colloids that can easily coagulate or settle.

For instance, in the given exercise, gum, starch, and gelatin exhibit high solvent affinity as they dissolve and form gels with water, showcasing how solvent affinity governs the formation and stability of colloids.

Stability in Colloids

Colloidal stability refers to the ability of a colloidal system to remain uniformly dispersed without settling or coagulating. This stability is influenced by factors such as solvent affinity, particle size, and the presence of stabilizing agents.

• **High stability** is common in lyophilic colloids due to strong solvation, preventing particles from clumping.
• **Low stability** often occurs in lyophobic colloids, requiring external stabilizers like electrolytes to prevent aggregation.

The choice of ingredients in formulations, like the inclusion of starch and gelatin in food products, utilizes their ability to stabilize colloids in water, maintaining texture and consistency. Stability is crucial for ensuring the uniformity and longevity of colloidal products in both natural and industrial settings.