Precipitation occurs when hydrated ferric oxide and
arsenious sulphide are mixed. It is due to

Options

A. Electroosmosis
B. Electrophoresis
C. Catalysis
D. Mutual coagulation

Answer

To solve the question regarding the precipitation that occurs when hydrated ferric oxide and arsenious sulfide are mixed, we will analyze the nature of the substances involved and the process leading to precipitation.

### Step-by-Step Solution:

1. **Identify the Compounds:**
- Hydrated ferric oxide can be represented as ferric hydroxide, \( \text{Fe(OH)}_3 \), which is a positively charged colloidal particle.
- Arsenious sulfide is represented as \( \text{As}_2\text{S}_3 \), which is a negatively charged colloidal particle.

2. **Understand the Nature of Precipitation:**
- Precipitation occurs when oppositely charged particles come together and neutralize each other, leading to the formation of a solid that separates from the solution.

3. **Analyze the Interaction:**
- When hydrated ferric oxide (positively charged) and arsenious sulfide (negatively charged) are mixed, they will attract each other due to their opposite charges.
- This attraction leads to mutual coagulation, where the particles aggregate and form a precipitate.

4. **Eliminate Incorrect Options:**
- **Electroosmosis:** This involves the movement of liquid through a porous material under the influence of an electric field and does not lead to precipitation.
- **Electrophoresis:** This is the movement of charged particles in a fluid under the influence of an electric field, which does not result in precipitation.
- **Catalysis:** This refers to the acceleration of a chemical reaction by a catalyst, which is not relevant to the precipitation process in this context.

5. **Conclusion:**
- The correct explanation for the precipitation occurring when hydrated ferric oxide and arsenious sulfide are mixed is due to **mutual coagulation** of the positively and negatively charged particles.

### Final Answer:
The precipitation occurs due to **mutual coagulation** of hydrated ferric oxide and arsenious sulfide.