JEE Main · 2021 · 26 Aug 2021 · Shift 2 · Surface Chemistry

A colloidal particle takes the charge of the ion it adsorbs preferentially, and that is the lattice ion present in excess in the medium. When silver nitrate is added to a potassium iodide solution the iodide is in excess, so the silver iodide particles adsorb ${\mathrm{I}}^{-}$ and the sol carries a negative charge. In the reverse order of addition the excess $\mathrm{A}{\mathrm{g}}^{+}$ is adsorbed and the sol is positive, while ferric hydroxide formed by hydrolysing $\mathrm{FeC}{\mathrm{l}}_3$ in hot water and hydrated alumina in water both adsorb cations and are positively charged. The sol with negatively charged particles is the one obtained by adding $\mathrm{AgN}{\mathrm{O}}_3$ to KI solution.