Answer the question on the basis of passage given below:

A colloid is a heterogenous system in which a substance is dispersed (dispersed phase) as fine particles in another substance called dispersion medium. Their particle size is intermediate between that of pure solutions (<1 nm) and suspension (>1000 nm).

They are known for interesting properties such as enormous surface area, Tyndall effect, Brownian Motion etc. They are widely used in industries and day to day life. Some prominent uses of colloids are those in smoke precipitators, purification of water, medicines cleansers etc.

Which of the following statements correctly explains the charge on hydrated ferric oxide sol, prepared by adding $FeCl_3$ to hot water?

It is positively charged due to preferential adsorption of $Fe^{3+}$ ions.

The correct answer is Option (2) → It is positively charged due to preferential adsorption of $Fe^{3+}$ ions.

The correct statement that explains the charge on hydrated ferric oxide sol, prepared by adding FeCl₃ to hot water, is: It is positively charged due to preferential adsorption of Fe³⁺ ions.

When FeCl₃ is added to hot water, hydrated ferric oxide sol (Fe₂O₃·xH₂O) forms. During the formation of the sol, the ferric oxide particles tend to preferentially adsorb Fe³⁺ ions from the surrounding solution.

Preferential adsorption: The ferric oxide particles (Fe₂O₃) have a tendency to adsorb Fe³⁺ ions from the dissociation of FeCl₃ in water. This creates an excess of Fe³⁺ ions on the surface of the sol particles, giving the colloidal particles a positive charge.

Charge stabilization: The positive charge on the sol particles is stabilized by the surrounding counterions (in this case, Cl⁻) which form a diffuse layer around the positively charged particles, maintaining electrical neutrality overall but with a positively charged colloidal particle.

Why the other options are incorrect:

Negatively charged due to OH⁻ adsorption: This is incorrect because Fe³⁺ ions are preferentially adsorbed, not OH⁻ ions.

Negatively charged due to electron capture: Ferric oxide sol doesn't gain electrons to acquire a negative charge; instead, it adsorbs Fe³⁺.

No charge due to equal amounts of opposite charges: The sol particles are positively charged, and the counterions simply form a stabilizing layer, not canceling the charge.

Conclusion:

The sol of hydrated ferric oxide is positively charged due to the preferential adsorption of Fe³⁺ ions from the FeCl₃ solution.