A colloidal solution is subjected to an electric field. The colloidal particles move toward the anode. In the coagulation of this solution using NaCl, BaCl$_2$ and AlCl$_3$ separately, which one is the correct order of coagulation power?
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Remember the Hardy-Schulze rule: for coagulation of colloids, the effective ions are those with charge opposite to the colloidal particles. Higher the valency of these effective ions, greater their coagulating power.