There are several examples, which reveal that the surface of a solid has the tendency to attract and retain the molecules of the phase with which it comes into contact. These molecules remain only at the surface and do not go deeper into the bulk. The accumulation of molecular species at the surface rather than in the bulk of a solid or liquid is termed adsorption. The molecular species or substance, which concentrates or accumulates at the surface is termed adsorbate and the material on the surface of which the adsorption takes place is called adsorbent.

Adsorption is essentially a surface phenomenon. Solids, particularly in finely divided state, have large surface area and therefore, charcoal, silica gel, alumina gel, clay, colloids, metals in finely divided state, etc. act as good adsorbents.

Which of the following statements is true with respect to the extent of physisorption?

Decreases with increase in temperature

The correct answer is option 4. Decreases with increase in temperature.

Physisorption involves the adsorption of molecules onto a surface due to weak van der Waals forces, such as dispersion forces, dipole-dipole interactions, and hydrogen bonding. The extent of physisorption refers to the amount of adsorbate (molecules being adsorbed) that adheres to the surface of the adsorbent (the material onto which adsorption occurs).

When it comes to the effect of temperature on physisorption:

Increase in Temperature: As the temperature increases, the kinetic energy of molecules also increases. This means that molecules have a higher tendency to move and collide with the surface. In the context of physisorption, higher temperatures provide molecules with enough energy to overcome the relatively weak van der Waals forces that drive adsorption. As a result, fewer molecules are adsorbed onto the surface because they can easily break away from the surface due to their increased kinetic energy.

Decrease in Temperature: Conversely, when the temperature decreases, the kinetic energy of molecules decreases as well. At lower temperatures, molecules have less energy to overcome the attractive forces between them and the surface. Therefore, more molecules tend to adhere to the surface through physisorption because they are less likely to escape due to their lower kinetic energy.

In summary, the extent of physisorption decreases with an increase in temperature because higher temperatures provide molecules with more kinetic energy, allowing them to overcome the weak van der Waals forces and reducing the amount of adsorbate adhering to the surface.