Adsorption arises due to the fact that the surface particles of the adsorbent are not in the same environment as the particles inside the bulk. The extent of adsorption increases with the increase of surface are per unit mass of the adsorbent at a given temperature and pressure. Another important factor featuring adsorption is the heat of adsorption. During adsorption, there is always a decrease in residual forces of the surface, therefore, it is invariably an exothermic process or \(\Delta H\) and \(\Delta S\) are always negative. There are mainly two types of adsorption of gases on solids. In physisorption, the attractive forces are mainly van der Waals forces while in cemisorption, actual bonding occurs between the particles of adsorbate and adsorbent. Generally, easily liquifying gases are adsorbed more easily on the surface of a solid as compared to the gases which are liquified with difficulty. Freundlich gave an emperical relationship between the quantity of gas adsorbed by unit mass of solid adsorbent and pressure, at a particular temperature.

Which of the following is not an example of adsorption?

Drying of the substance using anhydrous calcium chloride.

The correct answer is option 2. Drying of the substance using anhydrous calcium chloride.

Adsorption is the process by which molecules or particles accumulate on the surface of a solid or liquid. It is a surface phenomenon where the substance being adsorbed (adsorbate) adheres to the surface of the adsorbing material (adsorbent). Absorption is a process where a substance is taken up by the volume (bulk) of the material. In absorption, the absorbed substance (absorbate) penetrates into the body of the absorbent.

Analysis of Each Option

1. Retaining the color of the ink on the surface of the chalk stick

When you draw with a pen or marker on a chalk stick, the ink’s dye molecules adhere to the chalk’s surface. Chalk, being porous and having a large surface area, allows the dye particles to be held on its surface. This is a classic example of adsorption because the dye particles do not penetrate deep into the chalk but rather stay on the surface. The interaction involves the surface of the chalk attracting and holding the ink molecules.

2. Drying of the substance using anhydrous calcium chloride

Anhydrous calcium chloride is a hygroscopic substance, meaning it has a strong affinity for water. When it is exposed to moisture, it absorbs water molecules into its bulk structure, leading to the formation of hydrated calcium chloride (\(CaCl_2 \cdot xH_2O\)). This process is absorption, not adsorption. The water molecules are absorbed into the internal structure of the calcium chloride, where they chemically combine to form a hydrated compound. The interaction occurs within the bulk of the material, not just on its surface. This is not an example of adsorption.

3. Decolouration of crude sugar

During the refining of crude sugar, the solution is often passed through activated charcoal or bone char. The colored impurities (such as organic dyes and pigments) adhere to the large surface area of the activated charcoal. This is an example of adsorption. The color molecules from the crude sugar solution stick to the surface of the activated charcoal particles, removing the color from the solution. The process relies on the surface phenomenon of adsorption.

4. Drying of air in the presence of silica gel

Silica gel is commonly used as a desiccant to remove moisture from the air or other gases. The water vapor in the air gets adsorbed onto the surface of the silica gel particles. This is an example of adsorption. The moisture in the air is attracted to and held on the surface of the silica gel beads due to the large surface area and porous nature of silica gel. The process involves the interaction between the surface of the silica gel and the water molecules.

Summary

Option 1 (Ink on chalk), Option 3 (Decolouration of sugar), and Option 4 (Drying air with silica gel) are examples of adsorption because the substances involved adhere to the surfaces of the materials involved.

Option 2 (Drying with anhydrous calcium chloride) is an example of absorption because the water is absorbed into the bulk of the calcium chloride, forming a hydrated compound. This process involves the bulk material, not just the surface, distinguishing it from adsorption.

Conclusion

The key distinction is that adsorption is a surface phenomenon, whereas absorption involves the bulk of the material. Option 2 involves absorption, making it the correct answer as the process that is not an example of adsorption.