In a colloidal state the particle size of the dispersed phase range between 103 to 106 pm and colloidal sol is heterogeneous in nature. Based on the nature of interaction between the dispersed phase and dispersion medium colloidal sol are classified as lyophilic and lyophobic. Stability of sol is due to presence of charge on the sol particles and the neutralisation of the same is known as coagulation or precipitation.
Based on this answer the following.
Bredig's arc method is.
Answer & explanation
Correct answer: option 2
The correct answer is Option (2) → To prepare colloidal sol
Bredig’s arc method (electrical disintegration) is a method of preparing colloidal sols of metals such as gold, silver and platinum. An electric arc is struck between electrodes of the metal kept under the dispersion medium (cooled in ice). The intense heat vaporises the metal, and the vapour condenses into particles of colloidal size. The process involves both dispersion and condensation.
Why the other options are incorrect
Option (1) To coagulate colloidal sol — Incorrect. Coagulation is done by adding electrolytes, boiling or electrophoresis.
Option (3) To purify the colloidal sol — Incorrect. Purification is done by dialysis, electrodialysis or ultrafiltration.
Option (4) To prepare the micelles — Incorrect. Micelles form on their own when a surfactant such as soap is dissolved above the Kraft temperature and the CMC.
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):
Electrical Disintegration or Bredig’s Arc Method – “This process involves dispersion as well as condensation. Colloidal sols of metals such as gold, silver, platinum, etc., can be prepared by this method. In this method, electric arc is struck between electrodes of the metal immersed in the dispersion medium (Fig. 5.8). The intense heat produced vapourises the metal, which then condenses to form particles of colloidal size.”