The minimum milligrams of a protective colloid to be added to 10 mL red gold sol so that no coagulation takes place when 1 mL of `10%` NaCl solution is rapidly added to it is called its…………

Explanation

To solve the problem, we need to understand the concept of the "gold number" in colloid chemistry. The gold number is defined as the minimum amount of protective colloid (in milligrams) that must be added to a certain volume of a colloidal solution (in this case, red gold sol) to prevent coagulation when a specific amount of an electrolyte (here, NaCl) is added.

### Step-by-Step Solution:

1. **Understanding the Concept**:
- The gold number indicates the stability of a colloidal solution against coagulation. A lower gold number means that less protective colloid is needed to prevent coagulation.

2. **Identifying the Components**:
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