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.
Which of the following does not form a lyophilic colloid?
Answer & explanation
Correct answer: option 4
The correct answer is Option (4) → Metallic sulphides added to water
Lyophilic (liquid-loving) sols form directly when a substance such as gum, gelatine, starch, rubber or a protein is mixed with a suitable liquid; they are reversible and quite stable. Lyophobic (liquid-hating) substances, such as metals and their sulphides, do not form a sol when merely mixed with the dispersion medium; their sols have to be made by special methods and are easily coagulated by electrolytes.
Option (1) Rubber dissolved in benzene. — Forms a lyophilic colloid. Rubber disperses directly in benzene to give a lyophilic sol.
Option (2) White portion of the egg dissolved into water. — Forms a lyophilic colloid. Egg albumin is a protein; it forms a lyophilic sol in water.
Option (3) Starch in water — Forms a lyophilic colloid. Starch on warming with water gives a lyophilic sol.
Option (4) Metallic sulphides added to water — Does not form a lyophilic colloid (lyophobic). Metal sulphides do not form a sol when just mixed with water; their sols need special methods.
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):
Classification Based on Nature of Interaction between Dispersed Phase and Dispersion Medium – “… (i) Lyophilic colloids: The word ‘lyophilic’ means liquid-loving. Colloidal sols directly formed by mixing substances like gum, gelatine, starch, rubber, etc., with a suitable liquid (the dispersion medium) are called lyophilic sols.”
Lyophobic Colloids
