Q.Which of the following salts is most effective for coagulating ferric hydroxide sol? Na2SO4, FeCl3, K4[Fe(CN)6]

Lyophobic sols are unstable and easily coagulated; adding a lyophilic sol (protective colloid) coats the particles and stabilises them. Correct option (d).

Lyophobic (solvent-hating) sols are only weakly stabilised and coagulate readily. Adding a lyophilic (solvent-loving) sol makes the lyophilic particles form a protective layer around the lyophobic particles, preventing them from coming together - this is protective action (measured by the gold number).

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Coagulation is aggregation of colloidal particles; peptization is the opposite (breaking a precipitate into a sol). So the process not involving coagulation is peptization. Correct option (c).

Examining each:

• Use of NaCl to stop bleeding: electrolyte coagulates the colloidal blood -> coagulation.
• Formation of delta: river (colloidal clay) sol coagulated by electrolytes in sea water -> coagulation.
• Treatment of pond water with potash alum: Al3+ ions coagulate suspended colloidal impurities -> coagulation. …

Milk = fat droplets dispersed in water (O/W); butter = water droplets dispersed in fat (W/O).

An emulsion is a colloidal dispersion of one liquid in another immiscible liquid. There are two types, distinguished by which liquid forms the dispersed phase and which forms the continuous (dispersion) medium:

• Oil-in-water (O/W): oil (or fat) droplets are dispersed throughout a continuous water phase.
• Water-in-oil (W/O): water droplets are dispersed throughout a continuous oil/fat phase. …

Fe(OH)3 sol carries a positive charge, so negatively charged ions coagulate it, and the higher the charge on that ion, the more effective it is (Hardy-Schulze rule).

Colloidal Fe(OH)3 particles adsorb Fe3+ ions preferentially and are therefore positively charged. According to the Hardy-Schulze rule, the ion of opposite charge to the sol is responsible for coagulation, and its coagulating power increases sharply with its charge (roughly with the 6th power of the charge, empirically).

Comparing the anions available:

• Na2SO4 supplies SO4^2- (charge -2)
• FeCl3 supplies Cl- (charge -1) — and its Fe3+ cation would actually stabilise, not coagulate, the positive Fe(OH)3 sol …