Step 1: Understanding the Concept:
Colloids are stable because particles carry a charge and repel each other.
Adding an electrolyte introduces ions of opposite charge that neutralize the colloid, causing it to precipitate (coagulate).
The efficiency of an ion in causing this coagulation is called its "flocculating power".
Step 2: Detailed Explanation:
The Hardy-Schulze rule provides two key points:
1. The ions carrying a charge opposite to that of the sol particles are effective in causing coagulation.
2. The coagulating power of an ion increases significantly with the magnitude of its charge (valence).
For example, to coagulate a negatively charged sol (like \(As_2S_3\)), the flocculating power of cations is in the order: \(Al^{3+}>Mg^{2+}>Na^+\).
For a positively charged sol (like \(Fe(OH)_3\)), the order of anions is: \([Fe(CN)_6]^{4-}>PO_4^{3-}>SO_4^{2-}>Cl^-\).
Step 3: Final Answer:
The given rule is the Hardy-Schulze rule, matching option (D).