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A colloidal dispersion is placed in an electric field, and its particles migrate toward the anode. If the same sol is coagulated using NaCl, BaCl2, and AlCl3 solutions, what is the correct order of their coagulating ability?

1. NaCl > BaCl2 > AlCl3
2. BaCl2 > AlCl3 > NaCl
3. AlCl3 > BaCl2 > NaCl
4. BaCl2 > NaCl > AlCl3

Correct answer: AlCl3 > BaCl2 > NaCl

Solution

Migration to the anode means the sol is negatively charged, so cations cause coagulation. By the Hardy-Schulze rule, higher cation charge means greater coagulating power: Al3+ > Ba2+ > Na+, i.e. AlCl3 > BaCl2 > NaCl.

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