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 (As
2S
3) 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 As
2S
3 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
+, Ba
2+, and Al
3+. 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: Al
3+>Ba
2+>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).