In a colloidal state the particle size of the dispersed phase range between 103 to 106 pm and colloidal sol is heterogeneous in nature. Based on the nature of interaction between the dispersed phase and dispersion medium colloidal sol are classified as lyophilic and lyophobic. Stability of sol is due to presence of charge on the sol particles and the neutralisation of the same is known as coagulation or precipitation.
Based on this answer the following.
When \(KI\) solution is added to \(AgNO_3\) solution such that \(AgNO_3\) amount exceeds \(KI\) amount.
Answer & explanation
Correct answer: option 2
The correct answer is Option (2) → The precipitated AgI adsorbs Ag+ ions from KI solution and positively charged colloidal sol result
A precipitate adsorbs the ion common to it that is present in excess in the dispersion medium. When KI solution is added to \( \ce{AgNO3} \) so that \( \ce{AgNO3} \) is in excess, AgI is precipitated:
\( \displaystyle \ce{AgNO3(aq) + KI(aq) -> AgI(s) + KNO3(aq)} \)
The AgI particles then adsorb the excess Ag+ ions and become positively charged, AgI/\( \ce{Ag+} \). (The opposite case, \( \ce{AgNO3} \) added to excess KI, gives AgI/\( \ce{I-} \), a negative sol.)
Why the other options are incorrect
Option (1) ...adsorbs I− ions ... negatively charged — Incorrect. This happens only when KI is in excess; here \( \ce{AgNO3} \) is in excess, so few free \( \ce{I^{-}} \) ions remain.
Option (3) The precipitated KI adsorbs NO3−... — Incorrect. KI is soluble and does not precipitate; the precipitate is AgI, and \( \ce{NO3^{-}} \) is not common to it.
Option (4) No colloidal sol. formation takes place — Incorrect. A positively charged AgI sol does form.
Note 1: It seems that there is a mistake in the original NTA paper, which prints “adsorbs Ag+ ions from KI solution” when it should read “from AgNO3 solution”.
Note 2: 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 – …
