JEE Main 2021 · 26 February · evening · Single correct · +4 / −1
The nature of charge on resulting colloidal particles when FeCl₃ is added to excess of hot water is :
- (a) neutral
- (b) sometimes positive and sometimes negative
- (c) positive
- (d) negative
Answer and solution
Answer (c) positive
Solution
Chemistry ma’am · 3 steps
Hot water hydrolyses FeCl₃ to Fe(OH)₃, and the sol grabs Fe³⁺, its own kind of ion.
- Hydrolysis. In hot water, FeCl₃ hydrolyses and forms tiny particles of hydrated ferric oxide. These particles make up the colloid.
FeCl₃ + 3H₂O →(hot) Fe(OH)₃ (sol) + 3HCl
- Preferential adsorption. A colloidal particle adsorbs ions that are common to its own lattice. Water is in excess, so all of the FeCl₃ goes into making the sol, and the dissolved FeCl₃ that has not yet hydrolysed supplies Fe³⁺ ions. The Fe(OH)₃ particles take these up, written as Fe₂O₃ · xH₂O / Fe³⁺.
- So. Each particle carries adsorbed Fe³⁺, so the sol is positively charged, option (c).
The idea to keep. A sol adsorbs the ion it shares with its own lattice, and whichever ion is in excess decides the sign. FeCl₃ into excess water gives a positive sol. Excess NaOH gives an OH⁻-coated, negative sol.
Answer from the NTA final answer key. Solution by PrepWiser.