Which one is not the characteristic of chemisorption:

Options

A. Multilayer adsorption
B. Exothermic nature
C. Strong adsorption by adsorption sites
D. Irreversible

Explanation

To determine which option is not a characteristic of chemisorption, we need to analyze the characteristics of chemisorption itself.

### Step-by-Step Solution:

1. **Understanding Chemisorption**:
- Chemisorption is the process where a substance (adsorbate) forms a strong chemical bond with the surface of another substance (adsorbent). This bond is typically covalent or ionic in nature.

2. **Analyzing the Options**:
- **Option A: Multilayer adsorption**:
- Chemisorption typically occurs in a monolayer, meaning only one layer of adsorbate molecules can bond to the surface of the adsorbent. Once the surface is fully occupied, no further layers can form because there are no free valencies left for additional bonding. Thus, this option is not a characteristic of chemisorption.

- **Option B: Exothermic nature**:
- The formation of chemical bonds during chemisorption releases energy, making the process exothermic. Therefore, this is a characteristic of chemisorption.

- **Option C: Strong adsorption by adsorption sites**:
- Chemisorption involves strong interactions due to the formation of chemical bonds at specific adsorption sites. This is indeed a characteristic of chemisorption.

- **Option D: Irreversible**:
- Once chemisorption occurs, the chemical bonds formed are typically strong and difficult to break, making the process largely irreversible. This is also a characteristic of chemisorption.

3. **Conclusion**:
- Based on the analysis, the option that is **not** a characteristic of chemisorption is **Option A: Multilayer adsorption**. Chemisorption is characterized by monolayer adsorption.

### Final Answer:
**The option that is not a characteristic of chemisorption is: Multilayer adsorption (Option A).**

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