Q.Why does physisorption decrease with the increase of temperature?

Physisorption releases heat: $\text{Gas} + \text{Solid} \rightleftharpoons \text{Gas/Solid} + \text{Heat}$. Increasing the temperature supplies energy that helps molecules overcome the weak van der Waals attraction and leave the surface.

Step 1 – Physisorption is exothermic. Adsorption is accompanied by a decrease in the residual surface forces, releasing heat:

Step 2 – Apply Le Chatelier's principle. For an exothermic equilibrium, increasing the temperature opposes the change by favouring the endothermic (backward) direction — i.e. desorption.

Step 3 – Molecular picture. Physisorption is held only by weak van der Waals forces. At higher temperature the adsorbed molecules gain enough kinetic energy to overcome these weak forces and escape from the surface, so less gas remains adsorbed.

Treat adsorption as an exothermic equilibrium and apply Le Chatelier's principle; reinforce with the weak-force (van der Waals) molecular argument.

• Confusing with chemisorption, which can increase with temperature at first.
• Forgetting to state that adsorption is exothermic — the whole argument rests on it.
• Saying pressure, not temperature, is the cause.