The correct statement (S) pertaining to the adsorption of a gas on a solid surface is (are)

Options

A. Adsorption is always exothermic
B. Physisorption may transform into chemisorption at high temperature
C. Physisorption increases with increasing temperature but chemisorption decreases with increasing temperature
D. Chemisorption is more exothermic than physisorption, however it is very slow due to higher energy of activation.

Explanation

To determine the correct statements pertaining to the adsorption of a gas on a solid surface, we will analyze each statement one by one.

### Step-by-Step Solution:

1. **Statement 1: Adsorption is always exothermic.**
- **Analysis:** This statement is correct. Adsorption is a process that releases heat, meaning it is exothermic in nature. The heat released is generally less in physisorption (physical adsorption) compared to chemisorption (chemical adsorption).

2. **Statement 2: Physisorption may transfer into chemisorption at high temperature.**
- **Analysis:** This statement is also correct. Physisorption involves weak Van der Waals forces, while chemisorption involves the formation of chemical bonds. At high temperatures, the energy may be sufficient to convert physisorption into chemisorption as the weak forces can be overcome, leading to stronger chemical bonds.

3. **Statement 3: Physisorption increases with an increase in temperature, but chemisorption decreases with an increase in temperature.**
- **Analysis:** This statement is incorrect. Physisorption generally decreases with an increase in temperature because the increased kinetic energy of gas molecules allows them to escape from the solid surface. Chemisorption initially may increase with temperature due to the need for activation energy but will eventually decrease at very high temperatures due to the breaking of bonds.

4. **Statement 4: Chemisorption is more exothermic than physisorption; however, it is very slow due to high activation energy.**
- **Analysis:** This statement is correct. Chemisorption releases more energy than physisorption because it involves the formation of stronger chemical bonds. However, the process is slower due to the higher activation energy required to initiate the reaction.

### Conclusion:
The correct statements are:
- Statement 1: Adsorption is always exothermic.
- Statement 2: Physisorption may transfer into chemisorption at high temperature.
- Statement 4: Chemisorption is more exothermic than physisorption; however, it is very slow due to high activation energy.