select the correct statement (s) :

Options

A. A sol is prepared by addition of excess of `AgNO_(3)` solution in Kl solution . The charge likely to develop on colloidal particle is positive
B. the effects o pressure on physical adsortion is high if temperature is low
C. Ultracentrifugation process is used for pegaration of colloids
D. Gold number is the index for extent of gold plating done

Answer

To solve the question, we need to analyze each statement provided and determine its correctness based on our knowledge of surface chemistry, particularly colloidal systems.

### Step-by-Step Solution:

1. **Statement 1**: A sol is prepared by the addition of excess AgNO3 solution to the KI solution. The charge likely to develop on the colloidal particle is positive.
- When AgNO3 is added to KI, a reaction occurs that forms AgI (silver iodide), which is a colloidal particle. Since AgNO3 is in excess, the remaining Ag+ ions will impart a positive charge to the colloidal AgI particles. Therefore, this statement is **correct**.

2. **Statement 2**: The effect of pressure on physical adsorption is high if temperature is low.
- Physical adsorption is an exothermic process, and according to Le Chatelier's principle, increasing pressure favors the process that results in a decrease in volume. Since physical adsorption decreases the volume of gas, higher pressure will enhance the adsorption process, especially at lower temperatures. Thus, this statement is **correct**.

3. **Statement 3**: Ultracentrifugation is the process of preparation of colloids.
- Ultracentrifugation is primarily used for the purification and separation of colloidal particles, not for their preparation. It helps in isolating colloids from impurities or separating different colloidal particles based on their size and density. Therefore, this statement is **incorrect**.

4. **Statement 4**: Gold number is the index of the extent of gold plating done.
- The gold number refers to the minimum amount of protective colloid needed to prevent the coagulation of a gold sol when a certain amount of electrolyte (like NaCl) is added. It does not measure the extent of gold plating but rather the protective nature of colloids. Thus, this statement is **incorrect**.

### Conclusion:
The correct statements are:
- Statement 1: Correct
- Statement 2: Correct
- Statement 3: Incorrect
- Statement 4: Incorrect

Therefore, the correct options are **1 and 2 only**.