Question

Assertion(A): Colloidal silver iodide is prepared by adding silver nitrate in slight excess to a potassium iodide solution. When subjected to an electric field, the colloidal particles migrate to the anode.
Reason(R): Colloidal particles adsorb ions and thus become electrically charged.
A. Both (A) and (R) are correct and (R) is the correct explanation for (A)
B. Both (A) and (R) are correct but (R) is not the correct explanation for (A).
C. (A) is correct but (R) is incorrect
D. (A) is incorrect but (R) is correct
E. Both (A) and (R) are incorrect

Answer 1

Hint: Think about what compound is present in excess and which is the limiting one. Determine which ions will be adsorbed based on that. Consider the type of charge that will be attracted to the anode.

Complete step by step solution:
-A colloidal solution is to be prepared in this question. It is prepared by adding an excess of silver nitrate to a solution of potassium iodide. Once all the silver ions have combined with the iodide ions, there will still be some left over silver nitrate. This silver nitrate will be dissociated in the solvent and the excess silver ions will get adsorbed onto the colloidal particles of the silver iodide. This adsorption of the silver ions on the silver iodide colloidal particles will make the particles positively charged.
-When any current is passed through an electrolyte, we know that the cathode is negatively charged and will attract positively charged particles and the anode is positively charged and will attract negatively charged particles. Here, the colloidal particles have the positively charged silver ions adsorbed to them, so they will migrate to the cathode once the current starts flowing. From this, we can infer that the given assertion statement (A) is incorrect.
-Now, let us look at the reason statement (R). We have just seen that the colloidal particles become electrically charged only when they adsorb some of the excess ions that are present in the solvent. Thus, we can say that the reason statement (R) is true.

Hence, the correct answer to this question is ‘D. (A) is incorrect but (R) is correct’

Note: Remember that if the potassium iodide solution is present in excess instead of the silver nitrate solution, then the ions that will get adsorbed to the colloidal particles will be the iodide ions. This will make the particles negatively charged and attracted towards the anode.