Question

The dispersal phase in colloidal iron (III) hydroxide and colloidal gold is positively and negatively charged respectively. Which of the following statements is not correct?
A.Magnesium chloride solution coagulates the gold solution more readily than iron (III) hydroxide solution.
B.Sodium sulphate solution causes coagulation in both solutions.
C.Mixing the solutions has no effect.
D.Coagulation in both the solutions can be brought about by electrophoresis.

Answer 1

(1) The colloidal state of matter refers to that state of matter in which the size of the particles ranges from 1 to 1000 nm.
(2) In a colloidal system, the term dispersed phase is used to denote the substance which is distributed in the dispersion medium in the form of colloidal particles while the term dispersion medium is used to denote the medium in which the substance is dispersed in the form of colloidal particles.

Complete step by step answer:
It is given that the dispersed phase in colloidal iron (III) hydroxide is positively charged and that in colloidal gold is negatively charged. Coagulation is brought about by ions which have charge opposite to that of the colloidal particles.
Let us first consider the effect of magnesium chloride on these two colloidal solutions. Since the dispersed phase in iron (III) hydroxide is positively charged, it can be coagulated by negatively charged ions. Since dispersed phase in colloidal gold is negatively charged, it can be coagulated by positively charged ions. So, the ${\text{M}}{{\text{g}}^{{\text{2 + }}}}$ions of ${\text{MgC}}{{\text{l}}_{\text{2}}}$ will coagulate the gold solution and the ${\text{C}}{{\text{l}}^{\text{ - }}}$ions of ${\text{MgC}}{{\text{l}}_{\text{2}}}$ will coagulate the iron (III) hydroxide solution. But the charge on the cation ${\text{M}}{{\text{g}}^{{\text{2 + }}}}$ is twice the charge on the anion ${\text{C}}{{\text{l}}^{\text{ - }}}$ and since coagulating power increases with the increase in charge of the coagulating ion, the coagulation of gold solution by ${\text{MgC}}{{\text{l}}_{\text{2}}}$ will be faster than that of iron (III) hydroxide. Hence, statement A is correct.
Now, consider the effect of sodium sulphate ${\text{N}}{{\text{a}}_{\text{2}}}{\text{S}}{{\text{O}}_{\text{4}}}$ solution provides ${\text{N}}{{\text{a}}^{{\text{2 + }}}}$ ions which coagulate negative gold sol and ${\text{S}}{{\text{O}}_{\text{4}}}^{{\text{2 - }}}$ ions which coagulate positive iron (III) hydroxide sol. So, ${\text{N}}{{\text{a}}_{\text{2}}}{\text{S}}{{\text{O}}_{\text{4}}}$ solution can bring coagulation in both solutions. Hence, statement B is also correct.
Since the two given colloidal solutions have oppositely charged dispersed phases, the charge on one sol will be neutralized by the charge on the other sol and so there will be mutual coagulation. Hence, statement C is not correct.
In electrophoresis, electric current is passed through the colloidal solution in such a way that only the dispersed particles move and so electrophoresis can bring coagulation in both the sols. Hence, statement D is correct.

So, the only option which is not correct is statement C.

Note:
Two colloidal sols having oppositely charged dispersed phases can bring about mutual coagulation. This means if one colloidal sol has positively charged dispersed phase and the other has negatively charged dispersed phase, the charge provided by one colloidal sol can be neutralized by the charge on the other sol resulting in mutual coagulation.