Which statement is correct about physical adsorption? 

Options

A. It is highly specific
B. It is unimolecular layer adsorption
C. It depends on critical temperature of adsorbate
D. It is irreversible

Answer

To determine which statement is correct about physical adsorption, we will analyze each of the provided statements one by one.

1. **Statement A: It is highly specific.**
- Physical adsorption is not highly specific. This type of adsorption occurs due to Van der Waals forces, which are weak intermolecular forces that are not dependent on the type of adsorbate or adsorbent. Therefore, this statement is **incorrect**.

2. **Statement B: It is unimolecular layer adsorption.**
- Physical adsorption does not form a unimolecular layer; instead, it can form multiple layers of adsorbate on the adsorbent surface. This is because physical adsorption is due to weak attractions rather than strong chemical bonds, allowing for the accumulation of multiple layers. Thus, this statement is also **incorrect**.

3. **Statement C: It depends upon the critical temperature of the adsorbate.**
- This statement is true. The extent of physical adsorption is influenced by the critical temperature of the adsorbate. At temperatures above the critical temperature, the adsorbate may not condense and thus will not adsorb effectively. Therefore, this statement is **correct**.

4. **Statement D: It is irreversible.**
- Physical adsorption is generally reversible. The enthalpy of adsorption in physical adsorption is low, meaning that only a small amount of energy is required to desorb the adsorbate from the adsorbent. Hence, this statement is **incorrect**.

**Conclusion:**
The only correct statement regarding physical adsorption is **Statement C: It depends upon the critical temperature of the adsorbate**.