A colloid is a heterogeneous system in which one substance is dispersed as very fine particles in another substance. Their range of diameters is between 1-1000 nm. Based on dispersed phase and dispersion medium, colloids may be of different types like sol, gel, emulsion, aerosol, foam etc. Based on nature of interaction between two phases, sols are divided into Lyophilic and Lyophobic sols. Colloidal solutions exhibit various properties like Tyndall effect, Brownian movement and Electrophoresis. Stability of colloids is due to presence of charge. If somehow charge is removed, the particles will coagulate. Coagulation rate by various solutions containing ions, is decided by Hardy-Schulze rule. Everyday we come across many colloids. The meals we eat, clothes we wear, wooden furniture etc. are largely composed of colloids.
The coagulation of \(TiO_2\) sol can be effectively carried out by the solution containing
Answer & explanation
Correct answer: option 4
The correct answer is Option (4) → \([Fe(CN)_6]^{4-}\)
A sol is coagulated by ions carrying the charge opposite to that of the colloidal particles: the ions neutralise the charge, the particles come together and settle. By the Hardy–Schulze rule, the greater the valence of the coagulating ion, the greater its coagulating power.
\( \ce{TiO2} \) sol, like other metal oxide sols, is a positively charged sol, so the anions do the coagulating. Their charges are:
\( \ce{Cl^{-}} \) (1−) < \( \ce{SO4^{2-}} \) (2−) < \( \ce{PO4^{3-}} \) (3−) < \( \ce{[Fe(CN)6]^{4-}} \) (4−)
The ferrocyanide ion has the highest charge, so it coagulates \( \ce{TiO2} \) sol most effectively.
Why the other options are incorrect
Option (1) Cl− — Incorrect. A univalent anion has the lowest coagulating power of the four.
Option (2) PO43− — Incorrect. Trivalent, strong, but weaker than the tetravalent \( \ce{[Fe(CN)6]^{4-}} \).
Option (3) SO42− — Incorrect. Divalent, weaker than both \( \ce{PO4^{3-}} \) and \( \ce{[Fe(CN)6]^{4-}} \).
Note: This question is from the pre-rationalised NCERT book (2022-23). The topic is not there in the latest NCERT book (2026-27), and it is also out of the CUET 2026 syllabus.
NCERT (Pre-rationalised edition, 2022-23):
Charge on Colloidal Particles – “… Positively charged sols: Hydrated metallic oxides, e.g., Al2O3.xH2O, CrO3.xH2O and Fe2O3.xH2O, etc.; Basic dye stuffs, e.g., methylene blue sol; Haemoglobin (blood); Oxides, e.g., TiO2 sol.”
Coagulation or Precipitation – “… The ion responsible for neutralisation of charge on the particles is called the coagulating ion. A negative ion causes the precipitation of positively charged sol and vice versa. It has been observed that, generally, the greater the valence of the flocculating ion added, the greater is its power to cause precipitation. This is known as Hardy-Schulze rule. In the coagulation of a negative sol, the flocculating power is in the order: Al3+ > Ba2+ > Na+.
Similarly, in the coagulation of a positive sol, the flocculating power is in the order: [Fe(CN)6]4− > PO43− > SO42− > Cl−”