Correct Answer – D
When two or more ions are present in the disperison medium, preferential adsorption of the ion common to the colloidal particle usually takes place.
In the former case of stannic oxide, a small amount of stannic chloride, `SnCl_(4)` is formed and the positively charged `Sn^(4+)` ion is prefferred and sol is positively charge, In the latter cse, a small amount of sodium stanate, `Na_(2)SnO_(3)` is preferred and accordingly the sol is negatively charged. The chloride and sodium ions known as counter ions, are directed towards the liquid phase.
When silver nitrate solution is added to a slight excess of postassium iodide solution, the precipitated silver iodide adsorbs iodide ions (present in excess) from the dispersion medium and negatively charged colloidal solution results. But when `KI` solution is added to a sight excess of `AgNO_(3)` solution, positively charged sol result due to adsorption of `Ag` ions (present in excess) from dispersion medium.
`{:(AgI//I^(-),AgI//Ag^(+)),(“Negatively charged”,”Positively charged”):}`
However, if silver nitrate and sodium iodide are mixed in equivalent amounts, there is precipitation of silver iodide and no sol is formed due to absence of charges. Once again the ions preferred by colloidal particle are those which are common to them. Thus, in the former case, silver iodide prefers iodide ion and not sodium ion. Similarly in the latter case, it prefers silver ion and not nitrate ion.
The counter ion in each case is directed towards the liquid phase (dispersion medium).
If `FeCl_(3)` is added to excess of hot water, a positively charged sol of hydrated ferric oxide is formed due to adsorption of `Fe^(3+)` ions. But when ferric chloride is adde to `NaOH`, a negatively charged sol is obtained with adsorption of `OH^(-)` ions.
`underset(“Positively charged”)(FeO_(3).xH_(2)”O”//Fe^(3+)) , underset(“Negatively charged”)(Fe_(2)O_(3).x H_(2)”O”//”OH”^(-))`