JEE Main 2020 · 6 September · morning · Single correct · +4 / −1
Kraft temperature is the temperature :
- (a) below which the aqueous solution of detergents starts freezing.
- (b) above which the formation of micelles takes place.
- (c) below which the formation of micelles takes place.
- (d) above which the aqueous solution of detergents starts boiling.
Answer and solution
Answer (b) above which the formation of micelles takes place.
Solution
Chemistry ma’am · 4 steps
Micelles need two conditions: enough surfactant and enough warmth. Krafft temperature is the warmth part.
- What a micelle is. Soap and detergent molecules have a long non-polar tail and a polar head. Above a certain concentration they cluster in water with tails inside and heads outside. These clusters are micelles.
- Two thresholds. Micelles form only above the critical micelle concentration (CMC). They also form only above a particular temperature, the Krafft temperature Tₖ.
- Why temperature matters. Below Tₖ the surfactant does not dissolve well enough to reach the CMC, so it stays as single ions or comes out as a solid. Above Tₖ the solubility rises sharply and micelles form.
- So. The Krafft temperature is the temperature above which micelles form, which is option (b).
The idea to keep. Remember the pair for micelles: concentration above the CMC and temperature above the Krafft temperature. Both must be crossed, and you cross both by going higher.
Answer from the NTA final answer key. Solution by PrepWiser.