In a colloidal state the particle size of the dispersed phase range between 103 to 106 pm and colloidal sol is heterogeneous in nature. Based on the nature of interaction between the dispersed phase and dispersion medium colloidal sol are classified as lyophilic and lyophobic. Stability of sol is due to presence of charge on the sol particles and the neutralisation of the same is known as coagulation or precipitation.
Based on this answer the following.
The formation of micelles takes place.
Answer & explanation
Correct answer: option 1
The correct answer is Option (1) → Only above a particular temperature and above a particular concentration
Substances such as soaps and detergents behave as normal strong electrolytes at low concentration, but form aggregates (micelles, associated colloids) at higher concentration. Micelle formation takes place only above a particular temperature, the Kraft temperature (Tk), and above a particular concentration, the critical micelle concentration (CMC). For soaps the CMC is 10−4 to 10−3 mol L−1.
Why the other options are incorrect
Option (2) Only below a particular temperature and below a particular concentration — Incorrect. Both conditions are reversed.
Option (3) Only above Kraft temperature and below CMC — Incorrect. Below the CMC the surfactant stays as separate ions; it must be above the CMC.
Option (4) Only below Kraft temperature and above CMC — Incorrect. Below the Kraft temperature micelles do not form; it must be above it.
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):
Associated Colloids (Micelles)
