Question

In Bredig’s arc method some alkali is added because
(A) It increase electrical conductance
(B) To obtain molecular colloid
(C) To obtain colloidal particles of same size
(D) To stabilize the sol

Answer 1

Hint : The bredig’s arc method is also known as electrical disintegration. It is a method of preparation of colloidal solutions of metals like silver, gold or platinum. It consists of both dispersion and condensation. In this method, an electric arc is struck between the electrodes of metal which are immersed in the dispersion medium.

Complete Step By Step Answer:
In this method, an arc is struck between electrodes under the water surface which contains some amount of stabilizing agent like traces of potassium hydroxide. The intense heat of the arc vaporizes some amount of the metal which is then condensed under cold water. The water should be kept cold as an ice bath. The colloidal particles that are prepared are stabilized by adding a little amount of potassium hydroxide to it. But note that this method is not suitable when the dispersion medium is an organic liquid because charing takes place. Here, the purpose of potassium hydroxide is to stabilize the sol. Potassium hydroxide is the alkali used in bredig’s arc method. So, in bredig’s arc method, the purpose of the alkali is to act as a stabilizer of the sols.
Therefore, option D is the correct answer.

Note :
A sol is a colloid that is made up of very tiny solid particles in a continuous liquid medium. The stability of the colloidal sols is because of the presence of the charge on the particles of colloid. Similar charged particles of colloids repel each other. This prevents the coagulation of colloidal solution and hence provides stability to the colloidal solution.