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JEE Main levelSingle correct optionDifficulty 2 of 3el184

Assertion (A): A dilute solution of sodium laurate shows a steady fall in surface tension with concentration, but above a certain concentration the surface tension becomes almost constant. Reason (R): Beyond the critical micelle concentration, added surfactant molecules aggregate into micelles instead of crowding further at the surface.

  1. ABoth A and R are true, and R is the correct explanation of A
  2. BBoth A and R are true, but R is not the correct explanation of A
  3. CA is true but R is false
  4. DA is false but R is true

Answer: A

Surface tension falls while surfactant molecules adsorb at the air-water interface. Once the interface is saturated (at the CMC), extra molecules form micelles in the bulk, so the surface tension stays almost constant. R correctly explains A.
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