Question

The critical micelle concentration is defined as:
a) The concentration at which micellation starts.
b) The concentration at which micelle starts behaving like an electrolyte.
c) The concentration at which dispersed phase is separated from dispersed medium.
d) The concentration at which a colloid is converted to a suspension.

Answer 1

CMC is the concentration at which the micelle formation starts. It is a particular concentration, and the temperature at which it occurs is known as Kraft’s temperature.The soap molecule contains a hydrophilic and hydrophobic part in the aggregated molecules which surrounds the dirt particle and helps in cleansing action.

Complete answer:
Colloids are the heterogeneous mixture of substances where one particle (dispersed phase) is dispersed as very fine particles in another substance known as dispersed medium.
By the criteria of the type of particles in the dispersed phase, colloids are divided into multimolecular, macromolecular and associated colloids.
Multimolecular colloids are those which on dissolution aggregate to form smaller or larger molecules having a range of colloidal particles. For e.g. sulphur sol consists of particles having thousands of ${S_8}$ molecules.
Macromolecular colloids are those where some of the macromolecules on a particular solvent have a diameter of colloidal range and thus behave as a colloid. For e.g. starch solution.
Associated colloids are defined as the colloids which at a low concentration act like strong electrolytes but at higher concentration show properties of colloids. This is due to the formation of aggregates which are known as micelles.
Thus the micelle is formed only above a particular temperature called $Kraft({T_K})temperature$ and at a particular concentration known as Critical Micelle Concentration(CMC).
As on diluting these higher concentrated solutions the particles behave like normal electrolytes.
For e.g. in soaps which are $sodium$or $potassium$ salts of higher fatty acids, the$CMC = {10^{ - 4}} - {10^{ - 3}}mol{L^{ - 1}}$,

Hence the correct option is (a).

Note:
The cleansing action of soap helps to understand the proper working of micelle around the dirt particles in clothes, etc. The associated colloids should not be mixed with multimolecular colloids as in both the cases aggregates are formed.