Electro - osmosic is observed when :

Options

A. disperion medium partcles begins to move in an electric field
B. dispersed phase to move in an electric field
C. both (a) and (b)
D. none of these

Answer

To solve the question "Electro-osmosis is observed when:", we will analyze the options provided and understand the concept of electro-osmosis.

### Step-by-Step Solution:

1. **Understanding Electro-osmosis**:
- Electro-osmosis is a phenomenon that occurs in colloidal systems when an electric field is applied. It involves the movement of the dispersion medium (liquid) relative to the dispersed phase (solid particles).

2. **Analyzing the Options**:
- **Option A**: Dispersion medium particles begin to move in an electric field.
- **Option B**: Dispersed phase moves in an electric field.
- **Option C**: Both A and B.
- **Option D**: None of these.

3. **Identifying the Correct Option**:
- In electro-osmosis, it is primarily the dispersion medium (the liquid) that moves in response to the electric field. This movement occurs because the charged particles in the dispersed phase attract or repel the surrounding liquid, causing it to flow.
- On the other hand, when the dispersed phase moves in an electric field, this is referred to as electrophoresis, which is a different phenomenon.

4. **Conclusion**:
- Therefore, the correct answer to the question is **Option A**: "Dispersion medium particles begin to move in an electric field."

### Final Answer:
Electro-osmosis is observed when **dispersion medium particles begin to move in an electric field**.