Answer the question on the basis of passage given below:
A colloid is a heterogeneous 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?
Answer & explanation
Correct answer: option 2
The correct answer is Option (2) → It is positively charged due to preferential adsorption of Fe3+ ions.
When \( \ce{FeCl3} \) is added to excess hot water, it is hydrolysed to hydrated ferric oxide:
\( \displaystyle \ce{2FeCl3 + (x+3)H2O} \xrightarrow{\text{excess hot water}} \underset{\text{sol}}{\ce{Fe2O3.xH2O}} \ce{+ 6HCl} \)
The colloidal particles preferentially adsorb the ions common to their own lattice, here the \( \ce{Fe^3+} \) ions still present in the solution. The sol, written \( \ce{Fe2O3.xH2O} \)/\( \ce{Fe^3+} \), is positively charged.
Why the other options are incorrect
Option (1) It is negatively charged due to preferential adsorption of OH− from water. — Incorrect. A negative sol, \( \ce{Fe2O3} \)·xH2O/\( \ce{OH^{-}} \), is formed only when \( \ce{FeCl3} \) is added to NaOH solution, where \( \ce{OH^{-}} \) is in excess.
Option (3) It is negatively charged due to electron capture — Incorrect. Electron capture charges metal sols made by electro-dispersion (Bredig’s arc), not a sol made by hydrolysis.
Option (4) Due to equal amounts of opposite charges, it has no charge. — Incorrect. The particles of a sol all carry the same charge; that mutual repulsion is what keeps them dispersed.
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.”