Method by which lyophobic sol can be protected

Options

A. by addition of oppositely charged sol.
B. by addition of an electrolyte
C. by addition of lyophilic sol
D. by boiling

Explanation

To determine the method by which a lyophobic sol can be protected, we can analyze the options given:

### Step-by-Step Solution:

1. **Understanding Lyophobic Sols**:
- Lyophobic sols are colloidal systems that do not have a strong affinity for the dispersion medium. They tend to coagulate easily when disturbed or when certain substances are added.

2. **Analyzing the Options**:
- **Option 1: Addition of Oppositely Charged Salt**:
- When an oppositely charged salt is added to a lyophobic sol, it leads to coagulation. This is because the charges neutralize each other, causing the particles to aggregate and precipitate out of the solution. Therefore, this method does not protect the lyophobic sol.

- **Option 2: Addition of Electrolyte**:
- Similar to the first option, adding an electrolyte can also lead to coagulation of the lyophobic sol. Electrolytes can neutralize the charge on the colloidal particles, leading to their aggregation. Thus, this method also fails to protect the sol.

- **Option 3: Addition of Lyophilic Salt**:
- When a lyophilic sol is added to a lyophobic sol, the lyophilic particles can surround and stabilize the lyophobic particles. The lyophilic sol has a strong affinity for the dispersion medium, which helps to protect the lyophobic sol from coagulation by forming a protective layer around it.

- **Option 4: Boiling**:
- Boiling a lyophobic sol can lead to its destruction rather than protection. Heat can cause the particles to aggregate and precipitate, thus this method is not protective.

3. **Conclusion**:
- The correct method to protect a lyophobic sol is by the addition of a lyophilic sol. This creates a protective layer around the lyophobic particles, preventing coagulation.

### Final Answer:
The method by which a lyophobic sol can be protected is by the addition of a lyophilic salt.