The correct statement(s) pertaining to the adsorption of a gas on a solid surface is(are)
(i) adsorption is always exothermic
(ii) physisorption may transform into chemsisorption transform into chemisorption at high temperature
(iii) physisorption increases with increasing temperature but chemisorption decreases with increasing temperature .
(iv) chemisorption is more exothermic than physisorption , however it is very slow due to higher energy of activation.

Options

A. (i) and (iv) only
B. (ii) and (iii) only
C. (i),(ii) and (iv) only
D. All of these.

Answer

To solve the question regarding the correct statements pertaining to the adsorption of a gas on a solid surface, let's analyze each statement one by one.

### Step-by-Step Solution:

1. **Statement (i): Adsorption is always exothermic.**
- **Analysis:** Adsorption involves the interaction between gas molecules and the surface of a solid. When gas molecules adhere to a solid surface, energy is released in the form of heat. This is because the system moves to a lower energy state. Hence, this statement is **correct**.

2. **Statement (ii): Physisorption may transform into chemisorption at high temperature.**
- **Analysis:** Physisorption (physical adsorption) occurs due to weak Van der Waals forces and is favored at lower temperatures. Chemisorption (chemical adsorption), on the other hand, involves the formation of strong chemical bonds and typically occurs at higher temperatures. Under certain conditions, physisorption can transition into chemisorption when sufficient energy is provided (e.g., at high temperatures). Therefore, this statement is **correct**.

3. **Statement (iii): Physisorption increases with increasing temperature but chemisorption decreases with increasing temperature.**
- **Analysis:** This statement is **incorrect**. Physisorption decreases with increasing temperature because the weak forces of attraction can be overcome by thermal energy. Conversely, chemisorption generally increases with temperature as higher energy facilitates the formation of chemical bonds.

4. **Statement (iv): Chemisorption is more exothermic than physisorption; however, it is very slow due to higher energy of activation.**
- **Analysis:** This statement is **correct**. Chemisorption is indeed more exothermic than physisorption because it involves stronger interactions (chemical bonds) compared to the weaker interactions in physisorption. However, chemisorption typically has a higher activation energy, making it a slower process compared to physisorption.

### Conclusion:
Based on the analysis:
- Correct statements: (i), (ii), and (iv).
- Incorrect statement: (iii).

Thus, the correct answer is **C: Statements (i), (ii), and (iv) are correct**.