Which of the following can protect a gold sol from coagulation by NaCl solution?

Options

A. `Fe(OH)_(3)`
B. Gelatin
C. `As_(2)S_(3)`
D. None of these

Answer

To determine which substance can protect a gold sol from coagulation by NaCl solution, we need to understand the concepts of lyophobic and lyophilic sols, as well as the mechanism of coagulation.

### Step-by-Step Solution:

1. **Understanding Gold Sol**:
Gold sol is a colloidal suspension of gold particles in a liquid. It is classified as a lyophobic sol, meaning it has a low affinity for the dispersion medium and is more prone to coagulation.

**Hint**: Recall that lyophobic sols are less stable and can easily coagulate when exposed to electrolytes.

2. **Coagulation Mechanism**:
Coagulation occurs when particles in a colloid come together to form larger aggregates, which can settle out of the solution. In the case of gold sol, adding NaCl introduces Na⁺ ions, which can neutralize the negative charge on the gold particles, allowing them to come closer together and coagulate.

**Hint**: Remember that neutralization of charges leads to attraction between particles, resulting in coagulation.

3. **Role of Protective Colloids**:
To prevent coagulation, we can add a protective colloid. Protective colloids are typically lyophilic substances that can form a protective layer around the lyophobic particles, preventing them from coming together.

**Hint**: Think about substances that can create a barrier around the gold particles to maintain their stability.

4. **Examples of Protective Colloids**:
Common examples of lyophilic colloids include starch, gelatin, and certain proteins. These substances can adsorb onto the surface of the gold particles, creating a hydration layer that prevents coagulation.

**Hint**: Consider organic substances that have a high affinity for water and can stabilize colloids.

5. **Conclusion**:
To protect gold sol from coagulation by NaCl, we should add a lyophilic colloid such as starch or gelatin. These substances will form a protective layer around the gold particles, preventing them from neutralizing and coagulating.

**Final Answer**: Starch or gelatin can protect a gold sol from coagulation by NaCl solution.