JEE Main 2022 · 25 June · morning · Single correct · +4 / −1
Using very little soap while washing clothes, does not serve the purpose of cleaning of clothes, because :
- (a) soap particles remain floating in water as ions.
- (b) the hydrophobic part of soap is not able to take away grease.
- (c) the micelles are not formed due to concentration of soap, below its CMC value.
- (d) colloidal structure of soap in water is completely disturbed.
Answer and solution
Answer (c) the micelles are not formed due to concentration of soap, below its CMC value.
Solution
Chemistry ma’am · 4 steps
Soap cleans only through micelles, and micelles appear only above a minimum concentration, the CMC.
- How soap cleans. A soap ion such as RCOO⁻ has a long hydrophobic hydrocarbon tail and a hydrophilic ionic head. In water the ions cluster into micelles: tails point inwards and dissolve the grease, heads face the water outside.
- Need for CMC. Micelles form only when the soap concentration is above a certain value called the critical micelle concentration (CMC). Below the CMC, soap stays as separate ions and behaves like an ordinary electrolyte.
- Too little soap. With very little soap, the concentration stays below the CMC, so no micelles form. Without micelles, there is nothing to trap the oily dirt and carry it into the water, so the clothes do not get clean.
- So. The reason is (c): the soap concentration is below its CMC, so micelles are not formed.
The idea to keep. Soap is an associated colloid. It forms micelles, and so cleans, only above its CMC (and above the Kraft temperature). Below that, it is just a dilute electrolyte.
Answer from the NTA final answer key. Solution by PrepWiser.