Which of the following salt solutions would coagulate the colloid solution formed when is added to solution, at the fastest rate?
- A.10 mL of 0.1 mol dm
- B.10 mL of 0.15 mol dm
- C.10 mL of 0.1 mol dm
- D.10 mL of 0.2 mol dm
Show answer and solution
Correct option: D
Solution
When FeCl₃ is added to excess NaOH, the colloid formed is Fe(OH)₃ — but in NaOH medium (excess negative ions), the colloidal particles adsorb OH⁻ and become negatively charged.
By Hardy-Schulze rule, coagulation of a negative sol requires a cation, and coagulating power increases sharply with the charge of the cation (roughly as z⁶). So the higher the cation charge, the more effective the coagulant.
Among the options, AlCl₃ provides Al³⁺ (charge = +3), which has far greater coagulating power than Ca²⁺ (+2) or Na⁺ (+1). Although Ca₃(PO₄)₂ gives PO₄³⁻ (a trivalent anion), it's the cation that matters here for a negative sol, and Ca²⁺ is only divalent.
Therefore, AlCl₃ with its trivalent Al³⁺ coagulates the negatively charged Fe(OH)₃ sol at the fastest rate — option 4.
By Hardy-Schulze rule, coagulation of a negative sol requires a cation, and coagulating power increases sharply with the charge of the cation (roughly as z⁶). So the higher the cation charge, the more effective the coagulant.
Among the options, AlCl₃ provides Al³⁺ (charge = +3), which has far greater coagulating power than Ca²⁺ (+2) or Na⁺ (+1). Although Ca₃(PO₄)₂ gives PO₄³⁻ (a trivalent anion), it's the cation that matters here for a negative sol, and Ca²⁺ is only divalent.
Therefore, AlCl₃ with its trivalent Al³⁺ coagulates the negatively charged Fe(OH)₃ sol at the fastest rate — option 4.
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