JEE Main 2021 · 18 March · evening · Single correct · +4 / −1
The charges on the colloidal CdS sol and TiO₂ sol are, respectively :
- (a) positive and positive
- (b) positive and negative
- (c) negative and negative
- (d) negative and positive
Answer and solution
Answer (d) negative and positive
Solution
Chemistry ma’am · 3 steps
Sort each sol by its family: metal sulphides carry negative charge, metal oxides carry positive charge.
- Where charge comes from. A colloidal particle takes its charge mostly by adsorbing ions from the solution it is made in. It prefers ions it has in common with its own lattice.
Sol Family Ion adsorbed Charge CdS metal sulphide S²⁻ / HS⁻ left over from H₂S negative TiO₂ hydrated metal oxide metal cations / H⁺ positive As₂S₃ (for comparison) metal sulphide S²⁻ negative Fe(OH)₃ (for comparison) hydrated metal oxide Fe³⁺ positive - Apply the pattern. CdS belongs with As₂S₃ among the negative metal sulphide sols. TiO₂ belongs with Fe(OH)₃ and Al₂O₃.xH₂O among the positive metal oxide sols.
- So. The CdS sol is negative and the TiO₂ sol is positive, which is option (d).
The idea to keep. Learn the two families: metal oxide and hydroxide sols, and basic dyes, are positive. Metal sulphide sols, metal sols like Au and Ag, and acid dyes are negative.
Answer from the NTA final answer key. Solution by PrepWiser.