Problem 165 - A Complete Resource Book in Chemistry for JEE Main

Question

Which of the following phenomenon occurs when a chalk stick is dipped in ink? (a) Adsorption of solvent (b) Absorption and adsorption both of solvent (c) Absorption of solvent (d) Adsorption of coloured substance

Step-by-Step Solution

Answer

The phenomenon that occurs is (d) adsorption of colored substance.

Step 1: Understand Adsorption and Absorption

Adsorption is the process where molecules adhere to a surface. In contrast, absorption involves the substance being taken up into the volume of the material.

Step 2: Observe Effect on Chalk

When a chalk stick is dipped in ink, the colored particles of the ink only coat the surface of the chalk, which indicates adsorption.

Step 3: Analyze Ink Movement

If we carefully observe, the solvent (liquid part of the ink) is absorbed into the body of the chalk, whereas the dye or colored particles remain on the surface.

Step 4: Combine Observations

Since the dye molecules adhere to the surface and the solvent penetrates into the chalk, both adsorption and absorption occur, particularly focusing on the adsorption of the dye.

Step 5: Select Correct Answer

Considering all observed phenomena, the correct answer is that adsorption of the colored substance occurs, making option (d) correct.

Key Concepts

Adsorption

Adsorption is a surface phenomenon where molecules stick to the surface of a material. It is not absorbed into the material's volume, rather it happens primarily on the outer layer. Adsorption is typically influenced by factors such as surface area and temperature. For example, larger surface areas allow more molecules to adhere, enhancing adsorption. Chemical bonds or van der Waals forces can hold these molecules to the surface. Many everyday occurrences showcase adsorption. For example:

• Activated charcoal in water filters captures impurities via adsorption.
• Industrial catalysts depend on adsorption to increase effectiveness.

When a chalk stick is dipped in ink, the dye particles are adsorbed, coating the chalk with color.

Absorption

Absorption involves a material taking in particles and distributing them throughout its volume. Unlike adsorption, which is a surface activity, absorption extends within the entire structure of a material. An easy way to think about this is to imagine a sponge absorbing water.

• Absorption can be physical, like water absorption in sponges.
• It can also be chemical, as seen in chemical reactions where substances are taken in.

In the case of the chalk and ink interaction, the solvent is absorbed inside the chalk's porous structure, demonstrating the absorption process.

Surface Phenomena

Surface phenomena encompass all processes that occur at the boundary between two phases, such as solid-liquid or solid-gas. Adsorption is a key example, where molecules settle on surfaces.
Surface phenomena are crucial in numerous applications, including weathering of rocks and the functionality of cleaning agents. Knowledge of surface phenomena helps in:

• Developing effective detergents and cleaners.
• Enhancing catalysts in industrial reactions for better efficiency.

Understanding these concepts helps us comprehend how substances interact at surfaces, like the ink on chalk.

Chalk and Ink Interaction

When a chalk stick is dipped in ink, two main interactions happen: adsorption and absorption. The chalk's surface captures the colored ink particles through adsorption, visible as the outer color change.
Simultaneously, the liquid component (solvent) of the ink seeps into the porous chalk structure, showing absorption in action. Thus, the interaction of chalk and ink showcases both phenomena occurring simultaneously.
Using this knowledge helps explain its effects:

• The chalk appears colored due to the adsorption of dye.
• The inner structure of the chalk gets slightly heavier due to the absorbed solvent.

Such interactions illustrate the importance of distinguishing between surface and volumetric processes.