A colloid is a heterogeneous system in which one substance is dispersed as very fine particles in another substance called dispersion medium. Colloids are classified on the basis of physical state of dispersed phase and dispersion medium, nature of interaction between dispersed phase and dispersion medium and types of particles of dispersed phase. There are various methods for the preparation of colloidal solutions such as chemical methods, electrical disintegration and peptization. Colloidal solutions when prepared, generally contain excessive amount of electrolytes, and some other soluble impurities. The process used for reducing the amount of impurities to a requisite minimum is known as purification of colloidal solution. Purification of colloidal solution is carried out by various methods such as Dialysis, Electrodialysis, Ultrafiltration etc. Colligative properties, Tyndall effect, colour, Brownian movement, charge on colloidal particles, electrophoresis, coagulation are various properties exhibited by colloidal solution.
When FeCl3 is added to excess of hot water:
Answer & explanation
Correct answer: option 1
The correct answer is Option (1) → A positively charged sol of hydrated Ferric oxide is formed due to adsorption of Fe3+ ions.
Colloidal particles acquire their charge by preferential adsorption of an ion common to the particle, usually the ion present in excess. When \( \ce{FeCl3} \) is added to excess of hot water, it is hydrolysed to hydrated ferric oxide, and the particles adsorb the \( \ce{Fe^{3+}} \) ions still present:
\( \displaystyle \ce{FeCl3 + 3H2O -> Fe(OH)3\ (sol) + 3HCl} \)
\( \displaystyle \ce{Fe2O3.xH2O/Fe^3+}\quad(\text{positively charged sol}) \)
Why the other options are incorrect
Option (2) Negatively charged sol is obtained with adsorption of OH− ions. — Incorrect. This happens when \( \ce{FeCl3} \) is added to NaOH solution, where \( \ce{OH^{-}} \) is in excess, not to hot water.
Option (3) A negatively charged sol is formed due to adsorption of Fe3+ ions. — Incorrect. Adsorbing positive \( \ce{Fe^{3+}} \) ions makes the particles positive, not negative.
Option (4) A positively charged sol is formed by the adsorption of OH− ions. — Incorrect. Adsorbing \( \ce{OH^{-}} \) would make the particles negative.
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 – “… (b) If FeCl3 is added to excess of hot water, a positively charged sol of hydrated ferric oxide is formed due to adsorption of Fe3+ ions. However, when ferric chloride is added to NaOH a negatively charged sol is obtained with adsorption of OH− ions.”