its answer is 1st option. how?

​Q2. Which of the following will form negatively charged colloidal solution ?

        (1) 100 ml 0.1 M AgNO3 + 100 ml 0.1 M KI                     (2) 100 ml 0.2 M AgNO3 + 100 ml 0.1 M Kl

        (3) 100 ml 0.1 M AgNO3 + 100 ml 0.2 Kl                          (4) 100 ml 0.2 M AgNO3 + 200 ml 0.1 M Kl
 

The correct option is (3). 

The charge on the solution will be negative. When silver nitrate solution is added to potassium iodide solution (excess), the precipitated silver iodide adsorbs iodide ions from the dispersion medium and negatively charged colloidal solution results 

(at this stage KI is in excess, I– and being common to AgI).

 

  AgI  +   I–  →  (AgI)I–

Colloidal  in excess  Colloidal solution

 particle  in the medium  becomes negatively charged

 

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