Rate of physisorption increases with:

Decrease in temperature

The correct answer is option 2. Decrease in temperature.

Physisorption, or physical adsorption, involves the weak attraction of gas molecules to a solid surface due to van der Waals forces. These forces are relatively weak and do not involve the formation of new chemical bonds.

Key Factors Affecting Physisorption:

Temperature:

Nature of Physisorption: Physisorption is an exothermic process, meaning it releases heat when gas molecules adhere to a solid surface.

Effect of Temperature Increase: When the temperature increases, the kinetic energy of the gas molecules also increases. This added energy can overcome the weak van der Waals forces, causing the adsorbed molecules to desorb (leave the surface). As a result, the extent of physisorption decreases with increasing temperature.

Effect of Temperature Decrease: Conversely, when the temperature decreases, the kinetic energy of the gas molecules is reduced. Lower energy means that the van der Waals forces can more effectively hold the gas molecules on the surface, leading to an increase in the extent of physisorption. Therefore, physisorption is favored at lower temperatures.

Pressure:

Effect of Pressure Increase: Increasing the pressure of the gas increases the number of gas molecules in the vicinity of the solid surface, which can lead to more molecules being adsorbed. Higher pressure generally favors physisorption.

Effect of Pressure Decrease: Decreasing the pressure reduces the number of gas molecules available for adsorption, which would decrease the rate of physisorption.

Surface Area:

Effect of Surface Area Increase: A larger surface area provides more sites for gas molecules to adsorb, thus increasing the rate of physisorption.

Effect of Surface Area Decrease: A smaller surface area provides fewer sites for adsorption, decreasing the rate of physisorption.

Conclusion:

Temperature: The rate of physisorption increases with a decrease in temperature because lower temperatures favor the exothermic adsorption process. The gas molecules have less kinetic energy and are more likely to stay adsorbed on the surface due to the van der Waals forces.

Incorrect Options: Increasing temperature, decreasing pressure, or decreasing surface area would all lead to a reduction in the rate of physisorption.