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?

Colloidal iron(III) hydroxide, $$Fe(OH)_3$$, carries a positive charge on its sol particles, whereas a gold sol (often prepared by reduction of $$AuCl_4^-$$) carries a negative charge. Coagulation of a lyophobic sol takes place when its charge is neutralised by oppositely-charged ions (counter-ions). The effectiveness of the counter-ion increases rapidly with its valency (Schulze-Hardy rule).

Case A: Action of $$MgCl_2$$
For the negatively charged gold sol, the counter-ion is $$Mg^{2+}$$ (valency 2).
For the positively charged $$Fe(OH)_3$$ sol, the counter-ion is $$Cl^-$$ (valency 1).
Because a divalent ion is far more effective than a monovalent ion, the gold sol coagulates much more readily than the $$Fe(OH)_3$$ sol. Option A is therefore a correct statement.

Case B: Action of $$Na_2SO_4$$
For the positive $$Fe(OH)_3$$ sol, the counter-ion is $$SO_4^{2-}$$ (valency 2).
For the negative gold sol, the counter-ion is $$Na^+$$ (valency 1).
Given sufficient concentration, both sols can indeed be coagulated, although the $$Fe(OH)_3$$ sol will coagulate more quickly. Hence Option B is also correct.

Case C: Mixing the two sols
When a positive sol is mixed with an equal quantity of a negative sol, the oppositely charged particles neutralise each other. This destroys the double layer, eliminates the repulsive force between particles and leads to rapid coagulation (mutual precipitation). Therefore the statement “mixing the sols has no effect” is false.

Case D: Electrophoresis
If an electric field is applied, positively charged $$Fe(OH)_3$$ particles move towards the cathode and lose their charge; negatively charged gold particles move towards the anode and likewise discharge. Both therefore undergo coagulation by electrophoresis. Option D is correct.

The only statement that is NOT correct is Option C.

Option C which is: mixing the sols has no effect