Which one of the following will show Tyndall effect?

The correct answer is Option (4) → Aqueous solution of soap above critical micelle concentration

The Tyndall effect is the scattering of light by particles of colloidal size, which makes the path of the beam visible. Soap is an associated colloid: above the critical micelle concentration (CMC) its ions cluster into micelles of colloidal size, which scatter light. Below the CMC it behaves as an ordinary electrolyte.

• Option (1) Aqueous solution of NaCl — Incorrect. It is a true solution of small \( \ce{Na+} \) and \( \ce{Cl-} \) ions, too small to scatter light.

• Option (2) Aqueous solution of glucose — Incorrect. It is a true solution of glucose molecules, so it shows no Tyndall effect.

• Option (3) Aqueous solution of soap below critical micelle concentration — Incorrect. The soap is present as separate ions, not micelles, so it is a true solution and shows no Tyndall effect.

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) – “… The formation of micelles takes place only above a particular temperature called Kraft temperature and above a particular concentration called critical micelle concentration (CMC). On dilution, these colloids revert back to individual ions.”

Tyndall Effect – “… The Tyndall effect is due to the fact that colloidal particles scatter light in all directions in space. This scattering of light illuminates the path of beam in the colloidal dispersion.”