When dilute aqueous solution of \(KI\) (excess) is added to \(AgNO_3\) solution, the charge on the \(AgI\) colloidal, particles formed will be:
Answer & explanation
Correct answer: option 2
The correct answer is Option (2) → Negative
A precipitated colloid adsorbs the ion that is common to it and present in excess in the medium. When \( \ce{AgNO3} \) is added to excess KI:
\( \displaystyle \ce{AgNO3 + KI_{(excess)} -> AgI v + KNO3} \)
The AgI particles adsorb the excess I− ions, giving AgI/\( \ce{I^{-}} \), a negatively charged sol. What decides the charge is which ion is in excess, not the order in which the solutions are poured: with KI in excess this is the book’s first case (a); its second case (b), giving a positive sol, needs \( \ce{AgNO3} \) in excess.
Why the other options are incorrect
Option (1) Positive — Incorrect. A positive sol, AgI/\( \ce{Ag^{+}} \), forms only when \( \ce{AgNO3} \) is in excess, so that \( \ce{Ag^{+}} \) ions are adsorbed.
Option (3) Neutral — Incorrect. The particles adsorb \( \ce{I^{-}} \) ions preferentially, so they are not neutral; this charge keeps the sol stable.
Option (4) Both positive and negative — Incorrect. All particles of one sol carry the same kind of charge; that is why they repel one another and do not coagulate.
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 – …
