Answer the question on basis of passage given below:
Lyophobic sols like those of metals (Au, Ag, etc) are unstable and are easily precipitated by addition of electrolytes. However, it is observed that the addition of certain lyophilic colloids like gums, soaps, gelatine, etc. to lyophobic colloids (like a metal sol) render lyophobic colloids difficult to coagulate by the addition of electrolytes. The process is known as 'protection' and the lyophilic colloids are termed as Protective colloids. It is believed that the protective action of the lyophilic colloids is due to the covering up of the particles of the lyophobic colloid by those of the lyophilic colloid. Evidently, smaller the gold number of a protective colloid, the greater is its protective action.
Critical micelle concentration of soap solutions lie in the range of:
Answer & explanation
Correct answer: option 3
The correct answer is Option (3) → \(10^{-4}\) to \(10^{-3}\) \(M\)
Soaps and synthetic detergents are associated colloids. At low concentration they behave as normal strong electrolytes. Above a particular temperature (the Kraft temperature) and above a particular concentration, called the critical micelle concentration (CMC), their ions aggregate into micelles and the solution shows colloidal behaviour. On dilution the micelles break up into ions again.
For soaps the CMC lies between 10−4 and 10−3 mol L−1, that is, 10−4 to 10−3 M.
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) – “There are some substances which at low concentrations behave as normal strong electrolytes, but at higher concentrations exhibit colloidal behaviour due to the formation of aggregates. The aggregated particles thus formed are called micelles. These are also known as associated colloids. The formation of micelles takes place only above a particular temperature called Kraft temperature (Tk) and above a particular concentration called critical micelle concentration (CMC). On dilution, these colloids revert back to individual ions. Surface active agents such as soaps and synthetic detergents belong to this class. For soaps, the CMC is 10–4 to 10–3 mol L–1.”