Answer the following questions:

(a) Which of the following electrolytes is most effective for the coagulation of AgI/Ag⁺ sol?

- MgCl₂, K₂SO₄, K₄[Fe(CN)₆]

(b) What happens when a freshly precipitated Fe(OH)₃ is shaken with a little amount of dilute solution of FeCl₃.

(c) Out of sulphur sol and proteins, which one forms macromolecular colloids?

(a) Coagulation is the settling of colloidal particles. This can be carried out by the addition of an excess of electrolyte where the colloids interact with the ions at a charge opposite to the charge present on themselves.

To determine the most effective electrolyte for coagulation, we follow Hardy-Schulze rule which states that generally, the greater the valence of the flocculating ion added, the greater is its power to cause precipitation. AgI/Ag⁺ is a positive sol, so the flocculating or coagulating power is in the order [Fe(CN)₆]^4– > PO₄^3– > SO₄^2– > Cl^–. Hence, out of the given electrolytes, K₄[Fe(CN)₆] is the most effective.

(b) When a small amount of diluted solution of FeCl₃ is added to freshly precipitated Fe(OH)₃, a reddish-brown colloidal solution is obtained. This is due to the process of peptization.

(c) Peptization is stated as the process of converting a precipitate into colloidal sol by shaking it with dispersion medium in the presence of a small amount of electrolyte. Peptization forms a positively charged colloidal sol of Fe(OH)₃ where FeCl₃ is a peptizing agent.

Colloids can be classified into multi molecular, macromolecular and associated colloids on the basis of the type of particles depending upon the type of particles of the dispersed phase.

Macromolecular colloids form when the size of the macromolecules may be in the colloidal range. Sulphur sol forms multimolecular colloids as consists of particles containing a thousand or more of S₈ sulphur molecules, while proteins form macromolecular colloids due to their size and their stability in solution.