Problem 20 - A Complete Resource Book in Chemistry for JEE Main

Question

Which one of the following is an example of adsorption? (a) ammonia in contact with water (b) anhydrous \(\mathrm{CaCl}_{2}\) with water (c) silica gel in contact with water vapours (d) all of these.

Step-by-Step Solution

Answer

Option (c) is the example of adsorption.

Step 1: Understanding Adsorption

Adsorption is a process where molecules of a substance, such as gases, liquids, or dissolved solids, adhere to the surface of a solid or a liquid. It's different from absorption, where a substance is uniformly distributed throughout the volume of the absorber.

Step 2: Analyzing Option (a)

In option (a), ammonia dissolves in water. This is an example of absorption, not adsorption, as ammonia is distributed in the water rather than adhering to a surface.

Step 3: Analyzing Option (b)

In option (b), anhydrous \(\mathrm{CaCl}_{2}\) absorbs water to form a hydrate. This again is an example of absorption, as the water becomes evenly integrated rather than sitting on the surface.

Step 4: Analyzing Option (c)

In option (c), silica gel in contact with water vapors is an example of adsorption. The water vapors adhere to the surface of the silica gel without penetrating its volume, making it an adsorption process.

Step 5: Evaluate Option (d)

Option (d) suggests all options demonstrate adsorption. Based on our analysis, only option (c) is an example of adsorption. Therefore, (d) is incorrect.

Key Concepts

Absorption vs Adsorption

Understanding the difference between absorption and adsorption is crucial in surface chemistry. Absorption involves the uptake of a substance throughout the bulk of the material. Imagine a sponge soaking up water; the water spreads evenly throughout the sponge's entirety.
This process results in the absorbate being uniformly distributed in the absorber.
Adsorption, on the other hand, occurs when particles such as gases or molecules adhere solely to the surface of the material. In the case of silica gel and water vapors, the water molecules cling to the surface of the silica gel without penetrating deeply into its structure.
This difference is significant in many chemical processes as each has distinct applications and considerations. For example:

• Absorption: Utilized in cleaning up spills, where the liquid is soaked into a material.
• Adsorption: Used in air filters and purifiers where contaminants stick to the surface of activated carbon or similar materials.

Chemical Processes

Chemical processes often involve interactions at the molecular level, either through chemical reactions or physical manipulations such as adsorption.
In these processes, understanding how molecules interact with surfaces—or distributed within the material—guides how we can utilize materials for various applications.
Take, for instance,

• Catalysis: Often relies on adsorption processes where reactant molecules adhere to the surface of a catalyst.
• Moisture control: Using silica gel to adsorb moisture from the air, as seen in the given exercise.

These examples highlight how adsorption differs from absorption in practical settings.
While absorption modifies the entire volume, adsorption changes only the surface characteristics.

Surface Chemistry

Surface chemistry specifically studies the chemical phenomena that occur at interfaces of surfaces.
The process of adsorption is a central topic in surface chemistry because it involves interactions at the material's surface, which can drastically alter surface properties.
Understanding surface chemistry is vital because it explains why certain materials adsorb specific substances while others do not.
This interaction is critical in many areas:

• Surface modifications in materials science, which allow for the design of materials with desired adsorption properties.
• The development of sensors where a target molecule adsorbs to a detector surface triggering a response.
• Environmental applications like air and water purification, where contaminants adhere to the surface of filtration media.

Through these applications, the principles of surface chemistry exemplify their importance in both scientific research and daily life.