When the concentration of an adsorbent higher on the surface of adsorbent than in the adjoining bulk, the phenomenon is called

Options

A. Chemisorption
B. Physisorption
C. Positive adsorption
D. Negative adsorption

Explanation

To answer the question, we need to analyze the options given and understand the concept of adsorption in surface chemistry.

### Step-by-Step Solution:

1. **Understanding Adsorption**:
- Adsorption is the process in which atoms, ions, or molecules from a gas, liquid, or dissolved solid adhere to a surface. This creates a film of the adsorbate on the surface of the adsorbent.

2. **Types of Adsorption**:
- There are two main types of adsorption:
- **Physisorption**: This is a physical adsorption process where the adsorbate is held to the surface by weak van der Waals forces. It is generally reversible and occurs at lower temperatures.
- **Chemisorption**: This involves the formation of a chemical bond between the adsorbate and the adsorbent. It is usually stronger than physisorption and is often irreversible.

3. **Concentration on the Surface**:
- The question specifically mentions that the concentration of the adsorbate is higher on the surface of the adsorbent than in the adjoining bulk. This indicates that there is a preferential accumulation of the adsorbate on the surface.

4. **Identifying the Correct Term**:
- The phenomenon where the concentration of an adsorbate is higher on the surface than in the bulk is termed **positive adsorption**. This is because it describes the process where the adsorbate is accumulating at the surface, leading to a higher concentration there compared to the bulk.

5. **Eliminating Other Options**:
- **Chemisorption** and **physisorption** refer to the types of bonding rather than the concentration difference.
- **Negative adsorption** would imply that the concentration on the surface is lower than in the bulk, which is not the case here.

### Conclusion:
Thus, the correct answer to the question is **positive adsorption**.

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