Question

The greater the valence of the flocculating ion added, the greater is its power to cause precipitation of a colloid. This rule is:

• A. Hund's rule
• B. Pauling's rule
• C. Henry's rule
• D. Hardy - Schulze rule

Hint:

Remember that coagulating power is inversely proportional to the coagulation value (the minimum concentration of electrolyte required to cause precipitation). An ion with a high valence has massive coagulating power, meaning its required coagulation value is extremely small!

Answer 1

The Correct Option is D

Concept: Colloidal solutions carry a net electric charge on their dispersed phase particles, which keeps them stable by mutual electrostatic repulsion. Adding an electrolyte introduces oppositely charged ions that neutralize this surface charge, leading to aggregation, coagulation, or precipitation. The Hardy-Schulze rule states two key guidelines regarding this process:
• Only the ions carrying a charge opposite to that of the colloidal particles are effective in causing coagulation. These are called flocculating or active ions.
• The coagulating or flocculating power of an active ion increases sharply with an increase in its valence (charge magnitude).

Step 1: Analyzing the relationship between ion valence and flocculating power.
According to the rule, the higher the charge magnitude on the active ion, the more efficiently it neutralizes the double-layer potential around the colloid. For instance, to coagulate a negatively charged sol (like arsenic sulfide, \(\text{As}_2\text{S}_3\)), the active ions are cations. Their flocculating power increases drastically following the order of their valency: \[ \text{Al}^{3+} > \text{Mg}^{2+} > \text{Na}^+ \] Similarly, to coagulate a positively charged sol (like ferric hydroxide, \(\text{Fe(OH)}_3\)), the active ions are anions. Their flocculating power trends as: \[ [\text{Fe(CN)}_6]^{4-} > \text{PO}_4^{3-} > \text{SO}_4^{2-} > \text{Cl}^- \]

Step 2: Matching the statement with the given options.
Let us briefly review the other rules mentioned:
• Hund's rule: Dictates that electron pairing in degenerate subshells cannot occur until each orbital is singly occupied.
• Henry's rule: Relates the solubility of a gas in a liquid directly to the partial pressure of that gas above the liquid.
• Hardy-Schulze rule: Perfectly matches the definition linking flocculation power to ion valence. Therefore, the statement describes the Hardy-Schulze rule, corresponding to option (D).