Recall basic facts about adsorption

Adsorption of a gas on a solid surface can be of two types:

• Physisorption: due to weak van der Waals' forces
• Chemisorption: due to formation of chemical bonds

Key properties

• Adsorption is generally accompanied by decrease in residual forces at the surface, so it releases energy.
• Hence, adsorption is an exothermic process.
• Physisorption has low enthalpy of adsorption.
• Chemisorption has much higher enthalpy of adsorption and often requires activation energy.

Check each option

Option A: "Adsorption is always exothermic"

Adsorption lowers the energy of the system, so heat is evolved.

Therefore, A is correct.

Option B: "Physisorption may transform into chemisorption at high temperature"

At low temperature, gas molecules may first get physically adsorbed. On increasing temperature, if activation energy is supplied, chemical bond formation may occur and physisorption can change into chemisorption.

Therefore, B is correct.

Option C: "Physisorption increases with increasing temperature but chemisorption decreases with increasing temperature"

This is opposite to the general trend:

• Physisorption decreases with increasing temperature because it is exothermic.
• Chemisorption usually increases initially with temperature because activation energy is needed.

Therefore, C is incorrect.

Option D: "Chemisorption is more exothermic than physisorption, however it is very slow due to higher energy of activation"

Chemisorption involves stronger bond formation, so it is more exothermic than physisorption. It often has appreciable activation energy, making it slow especially at lower temperatures.

Therefore, D is correct.

Final answer

The correct statements are: $\boxed{A,\ B,\ D}$

Comparison with stored answer

Stored correct answer: $A, B, D$

This matches the derived answer.