Surface Chemistry

is positively charged sol. charge of anion is least and so KBr is least effective in coagulating

The coagulation (or flocculation) of a colloidal sol involves neutralizing the charge on the colloidal particles, causing them to aggregate and precipitate. (A) Nature of As₂S₃ Sol: Arsenic sulfide (As₂S₃) sol is a typical example of a negatively charged colloid. The negative charge usually arises from the preferential adsorption of sulfide ions (S^2-) onto the surface of the As₂S₃ particles. (B) Coagulation by Electrolytes: To coagulate a negatively charged sol, electrolytes containing positively charged ions (cations) are added. The cations act as counter-ions, neutralizing the negative charge on the sol particles. (C) Hardy-Schulze Rule: This rule states that the effectiveness of an ion in causing coagulation (its coagulating power) is directly related to the magnitude of its charge. For coagulating a negatively charged sol, the greater the positive charge on the cation, the greater its coagulating power. (D) Comparing the Ions: We are given the cations Na⁺, Ba²⁺, and Al³⁺. Their charges are +1, +2, and +3, respectively. (E) Order of Coagulating Power: According to the Hardy-Schulze rule, the coagulating power follows the order of the charge magnitude: Al³⁺>Ba²⁺>Na⁺ (F) Increasing Order: The question asks for the order in which the coagulating power increases. Therefore, we arrange the ions from the lowest coagulating power to the highest: Na⁺2+3+ This corresponds to option (B).