When an excess of a very dilute aqueous solution of KI is added to a…

The correct answer is Option (4) → AgI/I⁻ ⋮ K⁺

A precipitated colloidal particle adsorbs the ion common to it that is present in excess; this ion forms the fixed layer and gives the particle its charge, and ions of opposite charge from the medium form the diffused layer (Helmholtz electrical double layer).

    \( \ce{AgNO3} \) + KI → AgI + \( \ce{KNO3} \)

With KI in excess, \( \ce{I^{-}} \) ions are left in the medium. AgI adsorbs I⁻ (fixed layer) and becomes a negatively charged sol; the counter ions \( \ce{K^{+}} \) form the diffused layer: AgI/I⁻ ⋮ K⁺.

The sign of the sol depends on which ion is in excess. In the book’s second example (KI added to \( \ce{AgNO3} \)), \( \ce{AgNO3} \) is in excess, so \( \ce{Ag+} \) is adsorbed; here KI is in excess, so \( \ce{I-} \) is adsorbed.

Why the other options are incorrect

• Option (1) AgI/Ag⁺ ⋮ I⁻ — Incorrect. \( \ce{Ag^{+}} \) is adsorbed only when \( \ce{AgNO3} \) is in excess (positive sol); then the counter ion is \( \ce{NO3^{-}} \), since no free \( \ce{I^{-}} \) is left.

• Option (2) AgI/K⁺ ⋮ NO₃⁻ — Incorrect. \( \ce{K^{+}} \) is not an ion of AgI, so it is not adsorbed in the fixed layer.

• Option (3) AgI/NO₃⁻ ⋮ Ag⁺ — Incorrect. \( \ce{NO3^{-}} \) is not a common ion of AgI either; it cannot form the fixed layer.

Note: This question is from the pre-rationalised NCERT book (2022-23). The topic is not there in the latest NCERT book (2026-27), and it is also out of the CUET 2026 syllabus.

NCERT (Pre-rationalised edition, 2022-23):
Charge on Colloidal Particles – “… (a) When silver nitrate solution is added to potassium iodide solution, the precipitated silver iodide adsorbs iodide ions from the dispersion medium and negatively charged colloidal solution results. However, when KI solution is added to AgNO₃ solution, positively charged sol results due to adsorption of Ag⁺ ions from dispersion medium.”