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Here is a question from the former AIEEE 2005

The disperse phase in colloidal iron (III) hydroxide and colloidal gold is positively and negatively charged respectively. Which of the following statements is not correct?

(a) Mixing the sols has no effect

(b) Coagulation in both sols can be brought about by electrophoresis.

(c) Magnesium chloride solution coagulates the gold sol. more readily than the iron (III) hydroxide sol.

(d) Sodium sulphate solution causes coagulation in both sols.

It is a single answer correct, and it is obvious that (a) is false - hence is the answer. But the statement (c) also seems to be false (and therefore should also be an answer).

According to Hardy Schulze law,

Greater the valency of the flocculating ion, greater will be its coagulation power.

Doesn't that mean $\ce{Fe(OH)3}$ sol, having $\ce{Fe^{3+}}$ ions, will coagulate the gold sol more readily than $\ce{MgCl2}$ sol, which contains $\ce{Mg^{2+}}$ ions? Or is there any specific reason why the $\ce{MgCl2}$ sol coagulates the gold sol more readily?

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Option (c) is actually correct. Magnesium chloride does coagulate gold sol faster than ferric hydroxide, because ferric hydroxide does not coagulate in the first place.

Wikipedia mentions that ferric hydroxide is insoluble at $\ce{pH}=7$, so there aren't any $\ce{Fe^{3+}}$ ions to coagulate the gold sol anyway.

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