Which of the following statements about physical adsorption is not correct?

Options

A. It is usually monolayer
B. It is reversible in nature
C. It involves Van der Waals interactions between adsorbent and adsorbate
D. It involves small enthalpy of adsorption as compared to chemisorption

Explanation

To determine which statement about physical adsorption is not correct, we will analyze the characteristics of physical adsorption and compare them with the provided statements.

### Step-by-Step Solution:

1. **Understand the Concept of Physical Adsorption**:
- Physical adsorption, also known as physisorption, is a process where gas molecules adhere to a solid surface through weak Van der Waals forces.

2. **Identify Key Characteristics of Physical Adsorption**:
- **Multi-layer Formation**: Physical adsorption can occur in multiple layers, unlike chemisorption which typically forms a single layer.
- **Reversibility**: Physical adsorption is generally reversible, meaning that the adsorbate can be removed from the adsorbent without significant energy input.
- **Weak Forces**: The interaction between the adsorbent and adsorbate is primarily due to Van der Waals forces.
- **Enthalpy of Adsorption**: The enthalpy change (ΔH) during physical adsorption is relatively low, typically in the range of 20 to 40 kJ/mol, which is much lower than that of chemisorption (which is usually greater than 80 kJ/mol).

3. **Evaluate the Given Statements**:
- **Statement 1**: "It is usually monolayer."
- This statement is **incorrect** because physical adsorption is characterized by multi-layer formation.
- **Statement 2**: "It is reversible in nature."
- This statement is **correct**.
- **Statement 3**: "It involves Van der Waals interaction between adsorbent and adsorbate."
- This statement is **correct**.
- **Statement 4**: "It involves small enthalpy of adsorption as compared to chemisorption."
- This statement is **correct**.

4. **Conclusion**:
- The statement that is not correct regarding physical adsorption is **Statement 1**, which claims that it is usually monolayer.

### Final Answer:
The incorrect statement about physical adsorption is: **"It is usually monolayer."**