Which of the following is not correct regarding the adsorption of a gas on the surface of solid ?

Options

A. 1.On increasing pressure, adsorption keeps on increasing
B. 2.Enthalpy and entropy changes are negative
C. 3.Chemisorption is more specific than physisorption
D. 4.It is a reversible reaction.

Explanation

To determine which statement is not correct regarding the adsorption of a gas on the surface of a solid, we will analyze each option provided in the question.

### Step-by-Step Solution:

1. **Understanding Adsorption**:
- Adsorption is a surface phenomenon where gas molecules (adsorbate) adhere to the surface of a solid (adsorbent). The molecules do not penetrate into the solid but remain on its surface.

2. **Analyzing the First Option**:
- **Statement**: "On increasing pressure, adsorption keeps on increasing."
- **Analysis**: This statement is not entirely correct. While adsorption initially increases with pressure, it reaches a saturation point where the surface of the solid becomes fully occupied. Beyond this saturation pressure, further increases in pressure do not lead to an increase in adsorption. Thus, this statement is false.

3. **Analyzing the Second Option**:
- **Statement**: "Enthalpy and entropy changes are negative."
- **Analysis**: This statement is correct. Adsorption is an exothermic process, meaning that the enthalpy change (ΔH) is negative. Additionally, as gas molecules become adsorbed, their randomness decreases, leading to a negative entropy change (ΔS). Therefore, this statement is true.

4. **Analyzing the Third Option**:
- **Statement**: "Chemisorption is more specific than physisorption."
- **Analysis**: This statement is also correct. Chemisorption involves the formation of chemical bonds (covalent or ionic) between the adsorbate and adsorbent, which makes it specific to certain molecules. In contrast, physisorption does not involve chemical bonding and is less specific. Thus, this statement is true.

5. **Analyzing the Fourth Option**:
- **Statement**: "It is a reversible reaction."
- **Analysis**: This statement is misleading. Physisorption can be reversible, as the adsorbed gas can be desorbed easily under certain conditions. However, chemisorption is typically irreversible due to the formation of strong chemical bonds. Therefore, the statement is not universally true and can be considered incorrect in the context of chemisorption.

### Conclusion:
The statement that is **not correct** regarding the adsorption of a gas on the surface of a solid is:
- **"On increasing pressure, adsorption keeps on increasing."**