Question

On which of the following properties does the coagulating power of an ion depend?

• A. The magnitude of the charge on the ion alone.
• B. Both magnitude and sign of the charge on the ion.
• C. The sign of charge on the ion alone.
• D. Size of the ion alone.

Hint:

Coagulating power is inversely proportional to the coagulation value (the minimum concentration of electrolyte required to cause coagulation). An ion with a high charge has high power and a very low coagulation value.

Answer 1

The Correct Option is B

Concept:
The coagulation (precipitation) of a colloidal solution by an electrolyte is governed by the Hardy-Schulze Rule. Colloidal particles carry a specific electric charge (either positive or negative), and to coagulate them, an ion of the opposite charge must be added.

Coagulation is caused by the ion having a charge opposite to that of the colloidal particles. For example, a negatively charged sol (like As₂S₃) is coagulated by cations (Na⁺, Ba²⁺, Al³⁺), while a positively charged sol (like Fe(OH)₃) is coagulated by anions (Cl^-, SO₄^2-). Therefore, the sign of the charge determines if the ion will be effective at all.

According to the Hardy-Schulze rule, the greater the valency (magnitude of charge) of the coagulating ion, the greater is its power to cause precipitation.

For a negatively charged sol, the coagulating power follows the order:
Al³⁺ > Ba²⁺ > Na⁺

Conclusion: Since both the sign (to ensure opposition) and the magnitude (to determine strength) are critical, option (b) is correct.