Q.When NaCl solution is added to Fe(OH)3 colloidal solution then-

Coagulation of a charged sol is caused by the ion of opposite charge in the added electrolyte, and its effectiveness rises steeply with its charge (Hardy–Schulze rule) — so the trivalent $Al^{3+}$ in $AlCl_3$ coagulates a negative sol far more effectively than the divalent $Mg^{2+}$ in $MgCl_2$.

Hardy–Schulze rule

When an electrolyte is added to a lyophobic colloidal sol, coagulation (precipitation of the dispersed particles) is brought about by the ion carrying a charge opposite to that of the colloidal particles. The Hardy–Schulze rule states that the coagulating (flocculating) power of such an ion increases very sharply with its valency/charge — roughly with a high power of the charge for the critical coagulation concentration, meaning a small increase in ionic charge produces a large increase in coagulating power.

Applying it here

For a negatively charged sol, the active coagulating ion in each electrolyte is the cation:

• $MgCl_2$ supplies $Mg^{2+}$ (charge $+2$)
• $AlCl_3$ supplies $Al^{3+}$ (charge $+3$) …

By the Hardy–Schulze rule, the ion of higher charge opposite to the sol's own charge coagulates it most effectively; here that is the trivalent phosphate ion.

Ferric hydroxide sol, $Fe(OH)_3$, is a positively charged colloidal sol. Coagulation requires an oppositely charged ion (here, an anion) from the added electrolyte. By the Hardy–Schulze rule, the coagulating power of an ion increases sharply with its charge/valency. Of the ions supplied by $Na_3PO_4$ (i.e. $Na^+$ and $PO_4^{3-}$) …

NaCl is an electrolyte → its Cl⁻ ions coagulate the positive Fe(OH)₃ sol.

Fe(OH)₃ forms a positively charged sol (it adsorbs Fe³⁺). Adding an electrolyte such as NaCl provides oppositely charged Cl⁻ ions which neutralise the charge on the colloidal particles; the particles …