Colloids

Step 1: Concept
Ferric hydroxide ( ) is a positively charged sol. According to the Hardy-Schulze rule, it is coagulated by negative ions (anions).
Step 2: Analysis
Coagulating power increases with the increase in the valency of the coagulating ion.
Step 3: Comparison
- (from KBr) has a charge of -1.
- has a charge of -2.
- has a charge of -2.
- has a charge of -4.
Step 4: Conclusion
Since has the smallest charge, it is the least effective.
Final Answer: (A)

Step 1: Understanding the Concept:
Aerosol is a colloidal system.

Step 2: Detailed Explanation:
In an aerosol, the dispersed phase can be solid or liquid, but the dispersion medium is always gas. Examples: fog (liquid in gas), smoke (solid in gas).

Step 3: Final Answer:
Gas