Gold number is minimum in case of……………

Answer

To solve the question about which substance has the minimum gold number, we can follow these steps:

### Step-by-Step Solution:

1. **Understand the Concept of Gold Number**:
- Gold number is defined as the minimum amount of a protective colloid (in milligrams) required to prevent the coagulation of 10 ml of gold sol when 1 ml of 10% sodium chloride (NaCl) is added to it.
- It is a measure of the protective power of the colloid; lower gold numbers indicate higher protective power.

2. **Identify the Relationship**:
- The gold number is inversely proportional to the protective power of the colloid. This means that a lower gold number indicates a stronger protective effect against coagulation.

3. **Research Common Protective Colloids**:
- Some common protective colloids include gelatin, gum arabic, and casein. Each of these has a different gold number.

4. **Find the Gold Numbers of Common Colloids**:
- Gelatin: Gold number is approximately 0.005 to 0.01.
- Gum Arabic: Gold number is higher than that of gelatin.
- Casein: Gold number is also higher than that of gelatin.

5. **Conclusion**:
- Since gelatin has the lowest gold number (0.005 to 0.01), it is the best protective colloid among the ones mentioned. Therefore, the answer to the question is gelatin.

### Final Answer:
The substance with the minimum gold number is **gelatin**.

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