Which of the following is not correct

Options

A. Physical adsorption decreases in the increase in temperature.
B. Physical adsorption is multi layered
C. Activation energy of physical adsorption is very high
D. enthalpy change of physical adsorption is about `20Kj mol^(-1)`

Answer

To determine which statement is not correct regarding physical adsorption, we will analyze each statement step by step.

### Step 1: Analyze the first statement
**Statement:** Physical adsorption decreases with an increase in temperature.
**Analysis:** This statement is correct. Physical adsorption is an exothermic process, and as temperature increases, the kinetic energy of the adsorbate molecules increases, making it harder for them to adhere to the surface. Therefore, physical adsorption tends to decrease with an increase in temperature.

### Step 2: Analyze the second statement
**Statement:** Physical adsorption is multilayer.
**Analysis:** This statement is also correct. Physical adsorption can occur in multiple layers on the surface of the adsorbent. Unlike chemical adsorption, which typically forms a monolayer, physical adsorption can lead to the formation of several layers of adsorbate molecules.

### Step 3: Analyze the third statement
**Statement:** Activation energy of physical adsorption is very high.
**Analysis:** This statement is incorrect. The activation energy for physical adsorption is generally low because it involves weaker van der Waals forces. This allows adsorbate molecules to easily adhere to the surface without requiring a significant amount of energy.

### Step 4: Analyze the fourth statement
**Statement:** Enthalpy change of physical adsorption is about 20 kJ per mole.
**Analysis:** This statement is correct. The enthalpy change for physical adsorption is typically in the range of 20-40 kJ per mole, indicating that it is a relatively low-energy process compared to chemical adsorption.

### Conclusion
After analyzing all the statements, we conclude that the incorrect statement is the third one: "Activation energy of physical adsorption is very high."